Fossil plants, Vol. 1: [A text-book] for students of botany and geology

by A. C. Seward

IV. ALGAE.

1. [DIATOMACEAE. (Diatoms.)]
2. [CHLOROPHYCEAE. (Green algae.)]
3. [RHODOPHYCEAE. (Red algae.)]
4. [PHAEOPHYCEAE. (Brown algae.)]

The presence of chlorophyll is one common characteristic of the numerous plants included in the Algae. The generally adopted classification rests in part on an artificial distinction, namely the prevailing colour of the plant.

It must be definitely admitted, at the outset, that palaeobotany has so far afforded extremely little trustworthy information as to the past history of algae. Were we to measure the importance of the geological history of these plants by the number of recorded fossil species, we should arrive at a totally wrong and misleading estimate. By far the greater number of the supposed fossil algae have no claim to be regarded as authentic records of this class of Thallophytes. It has been justly said that palaeontologists have been in the habit of referring to algae such impressions or markings on rocks as cannot well be included in any other group. “A fossil alga,” has often been the dernier ressort of the doubtful student.

LARGE SEAWEEDS.

Before discussing our knowledge, or rather lack of knowledge, of fossil algae at greater length, it will be well to briefly consider the manner of occurrence and botanical nature of existing forms. In the sea and in fresh water, as well as in damp places and even in situations subject to periods of drought, algae occur in abundance in all parts of the world. We find them attaining full development and reproducing themselves at a temperature of −1° C. in the Arctic Seas, and again living in enormous numbers in the waters of thermal springs. Around the coast-line of land areas, and on the floor of shallow seas algae exhibit a remarkable wealth of form and luxuriance of growth. As regards habit and structure, there is every gradation from algae in which the whole individual consists of a thin-walled unseptate vesicle, to those in which the thallus attains a length unsurpassed by any other plant, and of which the anatomical features clearly express a well-marked physiological division of labour such as occurs in the highest plants.

The large and leathery seaweeds which flourish in the extreme northern and southern seas are plants which it is reasonable to suppose might well have left traces of their existence in ancient sediments. Sir Joseph Hooker, in his account of the Antarctic flora[223], investigated during Sir James Ross’s voyage in H.M. ships Erebus and Terror, has given an exceedingly interesting description of the gigantic brown seaweeds of southern latitudes. The trunks are described as usually 5–10 feet long, and as thick as a human thigh, dividing towards the summit into numerous pendulous branches which are again broken up into sprays with linear ‘leaves.’ Hooker records how a captain of a brig employed his crew for two bitterly cold days in collecting Lessonia stems which had been washed up on the beach, thinking they were trunks of trees fit for burning. On our own coasts we are familiar with the common Laminaria, the large brown seaweed with long and strap-shaped or digitate fronds which grows on the rocks below low-tide level. The frond passes downwards into a thick and tough stipe firmly attached to the ground by special holdfasts. A transverse section of the stalk of a fairly old plant presents an appearance not unlike that of a section of a woody plant. In the centre there is a well-defined axial region or pith consisting of thick walled, long and narrow tubes pursuing a generally vertical though irregular course, and embedded in a matrix of gelatinous substance derived from the mucilaginous degeneration of the outer portions of the cell-walls. The greater part of such a section consists, however, of regularly disposed rows of cells which have obviously been formed by the activity of a zone of dividing or meristematic elements. The occurrence of distinct concentric rings in this secondary tissue clearly points to some periodicity of growth which is expressed by the alternation of narrow and broader cells. In the Antarctic genus Lessonia, the stem reaches a girth equal to that of a man’s thigh, and in structure it agrees closely with the smaller stem of Laminaria. In these large algal stems, the cells are not lignified as in woody plants, and in longitudinal section they have for the most part the form of somewhat elongated parenchyma, differing widely in appearance from the tracheids or vessels of woody plants. At the periphery of the Laminaria stem, represented in fig. 29, there occur numerous and comparatively large mucilage ducts.

Fig. 29. A, Transverse section of the stipe of a Laminaria, slightly enlarged. B, A small piece of the tissue between the central ‘pith’ and ‘cortex’ showing the radially disposed secondary elements more highly magnified.

In certain algae of different families the thallus is encrusted with carbonate of lime, and is thus rendered much more resistant. The Diatoms, on the other hand, possess still more durable siliceous tests which are particularly well adapted to resist the solvent action of water and other agents of destruction. It is these calcareous and siliceous forms which supply the greater part of the trustworthy data furnished by fossil algae.

SCARCITY OF FOSSIL ALGAE.

It remains to consider some of the causes to which we may attribute the scarcity of fossil algae, and the possible sources of error which beset any attempt to describe or assign names to impressions and casts simulating algal forms.

In the first place, the delicate nature of algal cells is a serious obstacle to fossilisation. Even in plants in which the woody stems have been preserved by a siliceous or calcareous solution, we frequently find the more delicate cells represented by a mass of crystalline matter without any trace of the cell-walls being preserved. In such plants as algae, where the cell-walls are not lignified, but consist of cellulose or some special form of cellulose, which readily breaks down into a mucilaginous product, the tissues have but a small chance of withstanding the wear and tear of fossilisation.

The danger of relying on external form as a means of recognition is especially patent in the case of those numerous markings or impressions frequently met with on rocks, and which resemble in outline the thallus of recent algae. Among animals, such as certain Polyzoa, the flat branching body of various algae is closely simulated, and in other plants, such as the frondose liverworts, the same thalloid and branched form of body is again met with. Some of the much dissected Aphlebia leaves of ferns (e.g. Rhacophyllum species) bear a striking resemblance to fossil algae; and numerous other examples might be quoted. In palaeobotanical literature we find a host of names, such as Chondrites, Fucoides[224], Caulerpites and others applied to indefinite and indistinct surface markings which happen to resemble in shape certain of the better known genera of recent seaweeds.

The close parallelism in outward form displayed by different genera and families of algae is in itself sufficient argument against the use of recent generic names for fossils of which the algal nature is often more than doubtful. Were external form to be accepted as a trustworthy guide, in the absence of internal structure and reproductive organs, such a genus as Caulerpa[225] would afford material for numerous generic designations. A comparison of the different species of this Siphoneous green alga brings out very clearly the exceedingly protean nature of this interesting genus, and serves as one instance among many of the small taxonomic value which can be attached to external configuration. Caulerpa pusilla Mart. and Her., C. taxifolia (Vahl.), C. plumaris Forsk., C. abies-marina J. Ag., C. ericifolia (Turn.), C. hypnoides (R. Br.), C. cactoides (Turn.), C. scalpelli formis (R. Br.), and others clearly illustrate the almost endless variety of form exhibited by the species of a single genus of algae. We constantly find in the several classes of plants a repetition of the same form either in the whole or in the separate members of the vegetative body, and but a slight acquaintance with plant types should lead us to use the test of external resemblance with the greatest possible caution. To emphasize this danger may seem merely the needless reiteration of a self-evident fact, but there is, perhaps, no source of error which has been more responsible for the creation of numerous worthless species among fossil plants.

Fig. 30. 1. Rill-mark (after Williamson). 2. Trail made by a seaweed dragged along a soft plaster of Paris surface (after Nathorst). 3. Tracks made by Goniada maculata, a Polychaet (after Nathorst). 4. Burrow of an insect. 4a. Section of the gallery (after Zeiller).

There is, however, another category of impressions and casts of common occurrence in sedimentary rocks which requires a brief notice. Very many of the fossil algae described in text-books and palaeobotanical memoirs have been shown to be of animal origin, and to be merely the casts of tracks and burrows. A few examples will best serve to illustrate the identity of many of the fossils referred to algae with animal trails and with impressions produced by inorganic agency.

Dr Nathorst of Stockholm has done more than any other worker to demonstrate the true nature of many of the species of Chondrites, Cruziana, Spirophyton, Eophyton, and numerous other genera. In 1867 there were discovered in certain Cambrian beds of Vestrogothia, long convex and furrowed structures in sandstone rocks which were described as the remains of some comparatively highly organised plant, and described under the generic name Eophyton[226]. By many authors these fossils have been referred to algae, but Nathorst has shown that the frond of an alga trailed along the surface of soft plaster of Paris produces a finely furrowed groove (fig. 30, 2) which would afford a cast similar to that of Eophyton. The same author has also adduced good reasons for believing that the Eophytons of Cambrian rocks may represent the trails made by the tentacles of a Medusa having a habit similar to that of Polydonia frondosa Ag. Impressions of Medusae have been described by Nathorst from the beds in which Eophyton occurs; and the specimens in the Stockholm Museum afford a remarkable instance of the rare preservation of a soft-bodied organism[227]. By allowing various animals to crawl over a soft-prepared surface it is possible to obtain moulds and casts which suggest in a striking manner the branched thallus of an alga. The tracks of the Polychaet, Goniada maculata Örstd.[228], one of the Glyceridae, are always branched and very algal-like in form (fig. 30, 3). Many of the so-called fossil algae are undoubtedly mere tracks or trails of this type. In the fossil-plant gallery of the British Museum there are several specimens of small branched casts, clearly marked as whitish fossils on a dark grey rock of Lower Eocene age from Bognor; these were described by Mantell and Brongniart[229] as an alga, but there is little doubt of their being of the same category as the track shown in fig. 30, 3.

FOSSILS SIMULATING ALGAE.

The well-known half-relief casts met with in Cambrian, Silurian and Carboniferous rocks, and known as Cruziana or Bilobites, are probably casts of the tracks of Crustaceans. The impression left by a King-Crab (Limulus) as it walks over a soft surface affords an example of this form of cast. It has been suggested that some of the Bilobites may be the casts of an organism like Balanoglossus[230], a worm-like animal supposed by some to have vertebrate affinities. The resemblance between some of the lower Palaeozoic Bilobites and the external features of a Balanoglossus is very striking, and such a comparison is worth considering in view of the fact that soft-bodied animals have occasionally left distinct impressions on ancient sediments.

The literature on the subject of fossil algae versus inorganic and animal markings is too extensive and too wearisome to consider in a short summary; the student will find a sufficient amount of such controversial writing—with references to more—in the works quoted below[231].

In the Stockholm Museum of Palaeobotany there is an exceedingly interesting collection of plaster casts obtained by Dr Nathorst in his experiments on the manufacture of fossil ‘algae,’ which afford convincing proof of the value and correctness of his general conclusions.

The pressure of the hand on a soft moist surface produces a raised pattern like a branched and delicate thallus. The well-known Oldhamia antiqua Forbes and Oldhamia radiata Forbes[232], from the Cambrian rocks of Ireland may, in part at least, owe their origin to mechanical causes, and we have no sufficient evidence for including them among the select class of true fossil algae. Sollas[233] has shown that the structure known as Oldhamia radiata is not merely superficial but that it extends across the cleavage-planes. Oldhamia is recorded from Lower Palaeozoic rocks in the Pyrenees[234] by Barrois, who agrees with Salter, Göppert and others in classing the fossil among the algae. The photograph accompanying Barrois’ description does not, however, add further evidence in favour of accepting Oldhamia as a genus of fossil algae.

The burrows made by Gryllotalpa vulgaris Latr., the Mole-cricket, have been shown by Zeiller to bear a close resemblance to a branch of a conifer in half-relief (fig. 30, 4), or to such a supposed algal genus as Phymatoderma[235].

In fig. 30, 1, we have what might well be described as a fossil alga. This is merely a cast of a miniature river-system such as one frequently sees cut out by the small rills of water flowing over a gently-sloping sandy beach. A cast figured and described by Newberry as an alga, Dendrophycus triassicus[236], from the Trias of the Connecticut Valley, is practically identical with the rill-marks shown in fig. 30, 1. The cracks produced in drying and contracting sediment may form moulds in which casts are subsequently produced by the deposition of an overlying layer of sand, and such casts have been erroneously referred to algal impressions[237]. Dawson[238] has figured two good examples of Carboniferous rill-marks from Nova Scotia in his paper on Palaeozoic burrows and tracks of invertebrate animals.

RECOGNITION OF FOSSIL ALGAE.

Fig. 31. Chondrites verisimilis Salt. Wenlock limestone, Dudley. From a specimen in the British Museum (V. 2550). Slightly reduced.

The specimen represented in fig. 31 affords an example of a fairly well-known fossil from the Wenlock limestone, originally described by Salter as Chondrites verisimilis Salt, from Dudley[239]. He regarded it as an alga, and the graphitic impression agrees closely in form with the thallus of some small seaweeds. A closer examination of the fossil reveals a curious and characteristic irregular wrinkling on the graphite surface, which suggests an organism of more chitinous and firmer material than that of an alga.

A similar and probably an identical fossil is described and figured by Lapworth[240] in an appendix to a paper by Walter Keeping on the geology of Central Wales, under the name of Odontocaulis Keepingi Lap. and regarded as a dendroid graptolite. In any case we have no satisfactory grounds for including these fossils in the plant-kingdom.

How then are we to recognise the traces of ancient algae? There is no golden rule, and we must admit the difficulty of separating real fossil algae from markings made by animal or mechanical agency. The presence of a carbonaceous film is occasionally a help, but its occurrence is no sure test of plant origin, nor is its absence a fatal objection to an organic origin. While being fully alive to the small value of external resemblance, and to the numerous agents which have been shown to be capable of producing appearances indistinguishable from plant impressions, we must not go too far in a purely negative direction.

An important contribution to the subject of fossil algae has lately appeared by Prof. Rothpletz[241]. He deals more particularly with the much discussed Flysch[242] Fucoids of Tertiary age, and while refusing to accept certain examples as fossil algae, he brings forward weighty arguments in favour of including several other forms among the algae. He is of opinion that most of the main divisions of the algae are represented among the Flysch Fucoids, but considers that the Phaeophyceae are the most numerous.

Rothpletz’s work is chiefly interesting as illustrating the application of microscopic examination and chemical analysis to the determination of fossil algae. Although he makes out a good case in favour of restoring many of the Tertiary fossils to the plant kingdom, the material at his disposal does not admit of satisfactory botanical diagnosis.

No doubt some of the fossils from the Silurian and Cambrian rocks are true algae, and Nathorst has pointed out that such a species as Hall’s Sphenothallus angustifolius[243] may well be an alga. Additional examples might be quoted from Bornemann and other writers, but in view of the attempts which are sometimes made to trace the development of more recent plants to more than doubtful Lower Palaeozoic Algae, one must agree with Nathorst’s opinion,—“Je crois que l’on rend un bien mauvais service à la théorie de l’évolution, en essayant de baser l’arbre généalogique des algues fossiles sur des corps aussi douteux que les Bilobites, Crossochorda, Eophyton, etc.[244]”

There are many carbonaceous impressions on rocks of different ages which it is reasonable to refer to algal origin, and although such are of little or no botanical value, it may be a convenience to refer to them under a definite term. The comprehensive generic name Algites[245] has been suggested as a convenient designation for impressions or casts which are probably those of algae.

SUPPOSED FOSSIL ALGAE.

Some of the fossils described by Mr Kidston from British Carboniferous rocks as probably algae present an undoubted algal appearance, and might be placed in the genus Algites; but in some cases—e.g. Chondrites plumosa[246] Kidst. from the Calciferous Sandstone of Eskdale, one feels much more doubtful; in this particular instance the impressions suggest the fine roots of a water-plant.

The statement is occasionally made that the numerous fossil algae and the absence of higher plants in the older strata justify the description of the oldest rocks as belonging to the ‘age of algae.’ Such an assertion rests on an unsound basis, and is rather the expression of what might be expected than what has been proved to be the case. The oldest plants with which we are at all closely acquainted are of such a type as to forcibly suggest that in the lowest fossiliferous rocks we are still very far from the sediments of that age which witnessed the dawn of plant life.

Many of the obscure markings on rock surfaces which have been referred to existing genera of algae or described as new genera, are much too doubtful to be included even under such a comprehensive name as Algites. Space does not admit of further reference to determinations of this type which abound in palaeontological literature.

It would be very difficult to produce satisfactory evidence for the algal nature of many of the supposed fossil algae from Cambrian rocks[247]; there has been a special tendency to recognise algal remains in the oldest fossiliferous strata, due in part no doubt to the fallacy that in that period nothing higher than Thallophytes is likely to have existed. The so-called Phycodes referred to by Credner[248] as characteristic of the Cambrian rocks of the Fichtelgebirge (“Phycoden-Schiefer”) is probably of inorganic origin, and comparable to the genus Vexillum of Saporta[249] and other writers, which Solms-Laubach has described as being formed every day in the soft mud of our ponds where local currents are checked by branches and other obstacles[250]. There are several good specimens of Phycodes in the Bergakademie of Berlin and in the Leipzig Museum which, I believe, clearly demonstrate the absence of all satisfactory evidence of an algal origin.

We may next pass to a short description of a few representative types of algae, which may reasonably be classed under definite families, and accepted as evidence possessing some botanical value.

A. DIATOMACEAE (Bacillariaceae).

This family occupies a somewhat isolated position among the algae, and is best considered as a distinct subdivision rather than as a family of the Phaeophyceae or Brown algae, with which it possesses as a common characteristic a brown-colouring matter.

Single-celled plants consisting of a simple protoplasmic body containing a nucleus and brown colouring matter (diatomin) associated with the chlorophyll. The cell-wall is in the form of two halves, known as valves, which fit into one another like the two portions of a pill-box. The cell-wall contains a large amount of silica, and the siliceous cases of the diatoms are commonly spoken of as the valves of the individual, or the frustules. Diatoms exhibit a characteristic creeping movement, and are reproduced by division, also by the development of spores in various forms[251].

The recent members of the family have an exceedingly wide distribution, occurring both in freshwater and in the sea. Owing to the lightness of the frustules, they are frequently carried along in the air, and atmospheric dust falling on ships at sea has been found to contain large numbers of diatoms[252]. The siliceous valves are abundant in guano deposits, and they have been found also in association with volcanic material. Diatomaceous deposits are now being formed in the Yellowstone Park district; “they cover many square miles in the vicinity of active or extinct hot spring vents of the park, and are often three feet, four feet, and sometimes five to six feet thick[253].” The gradual accumulation of the siliceous tests on the floor of a fresh-water lake results in the formation of a sediment consisting in part of pure silica. Such deposits, often spoken of as kieselguhr or diatomite, and used as a polishing material, occur in many parts of Britain, marking the sites of dried-up pools or lakes. At the northern end of the island of Skye there occurs an unusually pure deposit of diatomite overlain by peat and turf, and extending over an area of fifty-eight square miles. Many of the individuals in this deposit were in all probability carried into the lake by running water, while others lived in the lake and after death their tests contributed to the siliceous deposit[254]. The late Dr Ehrenberg published numerous papers on diatomaceous deposits in different parts of the world, and in his great work, Zur Mikrogeologie[255], he gave numerous and beautifully executed illustrations of such siliceous accumulations. In many of the samples he figures one sees fragments of plant tissues, spores of conifers and ferns, associated with the diatom tests. The occurrence of the pollen grains of coniferous trees in lacustrine and marine deposits is not surprising in view of their abundance in Lake Constance and other lakes. It is stated that the pollen of conifers in the Norwegian fiords plays an important part in the nourishment of the Rhizopod Saccamina[256].

DIATOMACEOUS OOZE.

In the waters of the ocean diatoms are of frequent occurrence, and very widely distributed. Sir Joseph Hooker records the existence of masses of diatomaceous ooze over a wide area in Antarctic regions[257]. Along the shores of the Victoria Barrier, a perpendicular wall of ice, between one and two hundred feet above sea-level, the soundings were found to be invariably charged with diatom remains, and from the base of the ice-wall there appeared to be in process of formation a bank of these tests stretching north for a distance of 200 miles. The more extended researches conducted during the cruise of the Challenger have clearly proved the enormous accumulations of diatoms now being formed on the ocean-bed[258]. South of latitude 45° S. there is now being built up a vast deposit which may be eventually upraised as a fairly pure siliceous rock. From extreme northern latitudes Nansen has recently recorded the occurrence of these lowly organised plants. He writes,—“I found a whole world of diatoms and other microscopical organisms, both vegetable and animal, living in the fresh-water pools on the Polar drift-ice, and constantly travelling from Siberia to the east coast of Greenland[259].” In warmer latitudes diatoms abound in the surface waters, but there they are associated with numerous other forms of the Plankton vegetation. The waters of the Amazon carry with them into the sea large numbers of fresh-water forms, which are floated out to sea and finally added to the rock-building material which is constantly accumulating on the ocean floor[260]. No definite results have so far been obtained as to the geographical and bathymetrical distribution of marine diatoms.

The enormous number of recent species precludes any attempt to give a description of the better-known forms. It is more important for us to realize how common and widely distributed are the living genera. The hard and almost indestructible valves have been frequently found in a fossil condition, often forming thick and extensive masses of siliceous rock. From diatom-beds now forming in lakes and on the ocean-bed we pass to deposits such as those in Skye and elsewhere, which mark the site of recently dried-up sheets of water, and so to older rocks of Tertiary age formed under similar conditions. Among the many examples of diatomaceous deposits of Tertiary and Cretaceous age mention should be made of those of Berlin, Königsberg, Bilin in Bohemia, and Richmond in Virginia. The diatoms in the beds of Berlin are regarded as fresh-water, and those of Richmond as marine. It has been pointed out by Pfitzer that it is a comparatively easy matter to distinguish between fresh-water and marine forms of diatoms. The diatomaceous rocks of Bilin are known as polishing slates; they attain a thickness of 50 feet. In these, as in many other cases, the deposit has become cemented together as a hard flinty or glassy rock, in which the cementing material was formed by the solution of some of the diatom tests[261]. In many cases in which calcareous and siliceous rocks reveal no direct evidence of organic origin it is probable that they were originally formed by the accumulation of plants of which the structure has been completely obliterated by secondary causes. The genus Gallionella plays an important part in the composition of the Bilin beds. Occasionally impressions of leaves and other organic remains are found associated with the diatoms in the siliceous rocks. In the British Museum (Botanical department) a large block of white powdery rock is exhibited as an example of a diatomaceous deposit of Tertiary age from Australia. It is described as being largely made up of the tests of fresh-water diatoms, such as Navicula, Gomphonema, Cymbella, Synedra, and others.

FOSSIL DIATOMS.

The abundance of Diatoms in Cretaceous rocks of the Paris basin has recently been recorded by Cayeux[262]; it would seem that these algae had already assumed an important rôle as rock-builders in pre-Tertiary times. Cayeux points out that the silica of these Cretaceous diatomaceous frustules has often been replaced by carbonate of calcium.

In addition to the occurrence of Diatoms in the various diatomaceous deposits, their siliceous tests may occasionally be recognised in argillaceous or other sediments. Shrubsole and Kitton[263] have described several species of Diatoms from the London Clay of Lower Eocene age. In many localities in the London basin the clay obtained from well-sinkings presented the appearance of being dusted with sulphur-like particles of a dark bronze or golden colour which glistened in the sunlight. These yellow bodies have been found to be diatomaceous frustules in which the silica has been replaced by iron pyrites. The genus Coscinodiscus is one of the commonest forms recorded from the London Clay[264].

Without further considering individual examples of diatomaceous rocks we may briefly notice the general facts of the geological history of the family. As Ehrenberg pointed out several years ago, the Tertiary and Cretaceous species of diatoms show a very marked resemblance to living forms. In many cases the species are identical, and the fossil deposits as a whole seem to differ in no special respect from those now being built up.

With the exception of two species of Liassic Diatoms, no trustworthy examples of the Diatomaceae have been found below the Cretaceous series. The oldest known Diatoms were discovered by Rothpletz[265] among the fibres of an Upper Lias sponge from Boll in Württemberg. They occur as small thimble-shaped siliceous tests with coccoliths and foraminifera in the horny skeleton of Phymatoderma, a genus formerly regarded as an alga. Rothpletz describes two species which he includes in the genus Pyxidicula, P. bollensis and P. liasica. This generic name of Ehrenberg is used by Schütt[266] as a subgenus of Stephanopyxis.

Seeing how great a resemblance there is between the recent and Cretaceous species, and how many examples there are of Tertiary diatom deposits, it is not a little surprising that the past history of these plants has not been traced to earlier periods. In 1876 Castracane[267], an Italian diatomist, gave an account of certain species of diatoms said to have been found in a block of coal from Liverpool obtained from the English Coal-Measures. The species were found to be identical with recent forms. It is generally agreed that these specimens cannot have been from the coal itself, but that they must have been living forms which had come to be associated with the coal. The late Prof. Williamson spent many years examining thin sections and other preparations of coal from various parts of the world, but he never found a trace of any fossil diatom. There is no apparent reason why diatoms should not be found in Pre-Cretaceous rocks, and the microscopic investigation of old sediments may well lead to their discovery. Prof. Bertrand of Lille, who has devoted himself for some time past to a detailed microscopical examination of coal, informs me that he has so far failed to discover any trace of Palaeozoic diatomaceous tests.

BACTRYLLIUM.

The genus Bactryllium is often quoted in text-books as a probable example of a Triassic diatom. It was first described by Heer[268] from the Trias of Switzerland and North Italy, also from the neighbourhood of Heidelberg, and regarded as an extinct member of the Diatomaceae. Heer defined the genus as follows:

“Small bodies, with parallel sides, rounded at either end, the surface traversed by one or two longitudinal grooves.”

(fig. 32, C.) Several species have been figured by Heer from beds of Muschelkalk, Keuper and Rhaetic age. He describes the wall as thick and firm (fig. 32, C. ii.) and probably composed of silica, with a hollow interior. The specimen shown in fig. 32, C. was found in the Rhaetic beds, and named by Heer Bactryllium deplanatum; it has a length of 4·5 mm.; the surface is transversely striated and traversed by a single longitudinal groove. Stefani[269] has given reasons in favour of removing Bactryllium from the plant to the animal kingdom; he points out that the specimens are too large for diatoms, and moreover that they are asymmetrical in form and possessed a calcareous and not a siliceous shell. He would place the fossil among the Pteropods, comparing it with such genera as Cuvierina and Hyalaea. In view of Stefani’s opinion we cannot attach any importance to this supposed diatom, especially as it has generally been regarded as at best but an unsatisfactory genus.

Fig. 32. A, Lithothamnion mamillosum Gümb. (i) In section, (ii) surface view [after Gümbel. (i) × 320, (ii) nat. size. B, Sycidium melo Sandb. (i) Surface view, (ii) transverse section (after Deecke). C, Bactryllium deplanatum Heer. (i) Surface view, (ii) section, showing the thick wall and hollow interior (after Heer). D, Calcareous pebble from a lake in Michigan. Rather less than nat. size (after Murray).

B. CHLOROPHYCEAE (Green Algae).

Thallus unseptate, having the form of a vesicle or a variously branched coenocyte, which may or may not be encrusted with carbonate of lime, or of filaments composed of cells containing a single nucleus, or of cells in which more than one nucleus occurs; in other instances consisting of a plate of cells or a cell-mass. Asexual reproduction by zoospores and other reproductive cells; sexual reproduction by means of the conjugation of similar gametes or by the fertilisation of a typical egg-cell by a motile spermatozoid.

This family of algae is represented at the present day by numerous and widely distributed marine and fresh-water genera, as well as by genera growing in moist air or as endophytes in the tissues of higher plants[270].

Seeing how very few fossil forms have been described which have any claim to inclusion in this subdivision of the Algae, it is unnecessary to enumerate or define the various families of the Chlorophyceae. It is true that many species have been figured as examples of different genera of green algae, but few of these possess any scientific value. There is, however, one division of the Chlorophyceae, the Siphoneae, which must be treated at some length on account of its importance from a palaeobotanical and geological point of view.

a. Siphoneae.

Thallus consisting of simple or branched cells very rarely divided by septa, and containing many nuclei. In certain genera the branches form a pseudoparenchymatous tissue by their repeated branching, and as a result of the intimate felting together of the branched cells. Reproduction is effected either by the conjugation of similar gametes or by the fertilisation of an egg-cell.

Vaucheria and Botrydium are two well-known British genera of this order, but most of the recent representatives live in tropical and subtropical seas. The most striking characteristic feature of this division of the Chlorophyceae is the fact that the thallus of a siphoneous alga consists of an unseptate coenocyte; the plant may be extremely small and simple, or it may reach a length of several inches, but in all cases the body does not consist of more than one cell or coenocyte.

From a palaeontological standpoint the Siphoneae are of exceptional interest. It is impossible to do more than refer to a few of the living and fossil genera. There are numerous fossil representatives already known, and there can be little doubt that further research would be productive of valuable results.

As examples of the order, a few genera may be described belonging to the three families Caulerpaceae, Codiaceae, and Dasycladaceae.

α. Caulerpaceae.

Thallus unseptate, showing an extraordinary variation in the external differentiation of the plant-body. Reproduction is effected by means of detached portions of the parent plant.

The genus Caulerpa, represented by a few species in the Mediterranean and by many tropical forms, has already been alluded to as a striking example of a plant which appears under a great many different forms[271]. As a recent writer has said, “Nature seems to have shown in this genus the utmost possibilities of the siphoneous thallus[272],” Fragments of coniferous twigs, the tracks and burrows of various animals and other objects have been described by several authors as fossil species of Caulerpa. As an illustration of the identification of a very doubtful fossil as a species of Caulerpites, reference may be made to such a form as C. cactoides Göpp.[273] from Silurian and Cambrian rocks. There are several examples of this fossil in the Brussels Museum which probably owe their origin to some burrowing animal, and may be compared with Zeiller’s figures of the tunnels made by the mole-cricket (fig. 30, 4)[274].

Mr Murray, of the British Museum, has recently described what he regards as a trustworthy example of a fossil Caulerpa from the Kimeridge Clay near Weymouth[275]. Specimens of the fossil were first figured in a book on the geology of the Dorset coast as casts of an equisetaceous plant[276].

To this fossil Murray has assigned the name Caulerpa Carruthersi, and given to it a scientific diagnosis. The best specimens have the form of a slender central axis, giving off at fairly regular intervals whorls of short and somewhat clavate branches; they bear a superficial resemblance to such a recent species as Caulerpa cactoides Ag. An examination of several examples of this fossil leads me to express the opinion that there is not sufficient reason for assigning to them the name of a recent genus of algae[277]. To use the generic name of a recent plant without following the common custom of adding on the termination “ites” (i.e. Caulerpites) is as a general rule to be avoided in dealing with fossil forms; and there are, I believe, no satisfactory grounds for referring to these fossils as trustworthy examples of a Mesozoic alga.

In the present case I am disposed to regard the Caulerpa-like casts as of animal rather than plant origin. The clavate branches have the form of very deep moulds in the hard brown rock which have been filled in with blue mud. It is hardly conceivable that the branches of a soft watery plant such as Caulerpa could leave more than a faint impression on an old sea-floor. The specimens occur in different positions in the matrix of the rock and they are not confined to the lines of bedding; in none of the examples is there any trace of carbonaceous matter in association with the deep moulds. On the whole, then, this Kimeridge fossil cannot, I believe, be accepted as an authentic example of a Mesozoic Caulerpa.

It is not improbable that some of the supposed fossil algae may be casts of egg-cases or spawn-clusters of animals. In Ellis’ Natural History of the Corallines[278] there is a drawing representing a number of disc-like ovaries attached to a tough ligament, and referred to the mollusc Buccinum, which bears a certain resemblance to the Weymouth fossil. A similar body is figured by Fuchs[279] in an important memoir on supposed fossil algae.

It is not suggested that the Caulerpa Carruthersi of Murray should be regarded as the cast of some molluscan egg-case attached to a slender axis, but it is important to bear in mind the possibility of matching such extremely doubtful fossils with other organic bodies than the thallus of a Caulerpa. In an example of an egg-case in the Cambridge Zoological Museum, referred to a species of Pyrula, there is a hard, long and slender axis, bearing a series of semicircular chambers divided into radial compartments. The whole is hard and horny and might well be preserved as a fossil.

β. Codiaceae.

The members of this Order present a considerable diversity of form as regards the shape of the plant-body; the thallus of some species is encrusted with carbonate of lime. The order is widely distributed in tropical and temperate seas.

Among the recent genera Penicillus and Codium may be chosen as important types from the point of view of fossil representatives.

Codium.

The thallus of Codium consists of a spongy mass of tubular cell-branches which are differentiated into two fairly distinct regions, an outer peripheral layer in which the branches have long club-shaped terminations, and an inner region consisting of loosely interwoven filaments.

Codium Bursa L. and C. tomentosum Huds. are two well-known British species, the former presents the appearance of a spongy ball of cells, and in the latter the thallus is divided up into dichotomously forked branches[280]. In this genus the thallus is not encrusted with carbonate of lime, at least in recent species.

Sphaerocodium. Fig. 37, D.

Rothpletz[281] instituted this genus for certain small spherical or oval bodies varying from 1 mm. to 2 cm. in diameter, which have been found on crinoid stems or shell fragments of Triassic age. Each spherical body consists of dichotomously branched single-celled filaments, between 50 and 100µ in breadth, and from 300–500µ in height. The tubular cavities occasionally swell out into spherical spaces which are regarded by Rothpletz as sporangia.

There is not sufficient evidence that Sphaerocodium Bornemanni Roth. has been correctly referred to the Codiaceae. The sporangia-like swellings described by the author of the species are not by any means conclusive as characters of important taxonomic value. Figure 37, D, illustrates the general structure of the fossil as seen in a transverse section of one of the calcareous grains.

Like Girvanella, which has been referred by some writers to the Siphoneae, Sphaerocodium occurs in the form of oolitic grains. In the Triassic Raibler and St Cassian beds of the Tyrol, as well as in rocks of Rhaetic age in the Eastern Alps, it makes up large masses of limestone. Rothpletz compares the structure of this genus with that of the recent alga Codium adhaerens Ag., but it is wiser to regard such tubular structures as Girvanella, Siphonema[282] and Sphaerocodium as closely allied organisms, which are probably algae, but too imperfectly known to be referred to any particular family.

Penicillus.

The recent genus Penicillus is one of those algae formerly included among animals. Fig. 33, O, has been copied from a drawing of a species of Penicillus given by Lamouroux[283] under the generic name of Nesea in his treatise on the genera of Polyps published in 1821. He describes the genus as a brush-like Polyp with a simple stem.

The thallus consists of a stout stem terminating in a brush-like tuft of fine dichotomously-branched filaments. The apical branches are divided by regular constrictions into short oval or rod-like segments which may be encrusted with carbonate of lime. A few of the segments from the terminal tuft of a recent Penicillus are shown in fig. 35, E. Each of these calcareous segments has the form of an oval shell perforated at each end, and the wall is pierced by numerous fine canals. Penicillus is represented by about 10 recent species, which with one exception live in tropical seas.

The recognition of Penicillus, or a very similar type, in a fossil condition is due to Munier-Chalmas[284]. This keen observer has rendered great service to palaeobotany by directing attention to the calcareous algae in the Paris basin beds, and by proving that many of the fossils from these Tertiary deposits have been erroneously included by previous writers among the Foraminifera[285]. It is greatly to be desired that Prof. Munier-Chalmas may soon publish a monograph on the fossil Siphoneous forms of which he possesses a unique collection.

Ovulites. Figs. 33, K, L, and 35, F.

In his Natural History of Invertebrate Animals, Lamarck described some small oval bodies from the Calcaire Grossier (Eocene) of the Paris basin under the name of Ovulites. He defined them as follows:—“Polypier pierreux, libre, ovuliforme ou cylindracé, creux intérieurement, souvent percé aux deux bouts. Pores très petits, régulièrement disposés à la surface[286].”

Fig. 33. A and B, Cymopolia barbata (L.); A, transverse section of the calcareous cylinder. B, verticillate branches and sporangium after removal of the calcareous matrix (A and B after Munier-Chalmas). C and D, Acicularia Andrussowi Solms (C, after Andrussowi; D, after Solms). E, Acicularia Miocenica Reuss; section of a spicula (after Reuss). F and G, Acicularia sp. (after Carpenter), F × 40; G × 20. H, Acicularia Schencki (Möb.) (after Solms). I, Acetabularia Mediterranea Lamx.; section of the cap (after Falkenberg). K and L, Ovulites margaritula (Lam.) (after Munier-Chalmas); K slightly enlarged; L, a piece of the thallus more highly magnified. M, Cymopolia barbata (L.) (after Ellis, nat. size). N, C. barbata (L.); the surface of the thallus; magnified. O, Penicillus pyramidalis (Lamx.) (after Lamouroux, nat. size).

The specimens are referred to two species, Ovulites margaritula and O. elongata.

By some subsequent writers[287] these calcareous fossils, like miniature birds’ eggs with a hole at either end, were included among the Zoophytes. Carpenter and others afterwards referred Ovulites to the Foraminifera, and compared the genus with Lagena[288]. The single specimens of Ovulites have a length of 2–6 mm. At each end there is usually a fairly large and somewhat irregular hole (fig. 35, F), and in some rarer cases there may be two apertures at the broader end of an Ovulite. A good example of Ovulites margaritula with two pores at the broader end is figured by Michelin[289]. The surface of the shell when seen under a low magnifying power appears to be covered over with regularly arranged circular pores, which are the external openings of fine canals (fig. 33, L).

In 1878 Munier-Chalmas expressed the opinion, which was supported by strong evidence, that Ovulites should be referred to the siphoneous algae[290]. He regarded it as generically identical with Penicillus (Coralliodendron, Kützing). It has already been pointed out that in Penicillus the apical tuft of filaments is partially calcareous (fig. 33, O)[291]. The individual calcareous segments agree almost exactly with the fossil Ovulites. As a rule the Ovulites occur as separate egg- or rod-like bodies, but Munier-Chalmas informs me that occasionally two or three have been found joined end to end in their natural position. The terminal holes in the fossil specimens represent the apertures left after the detachment of the calcareous segments from the uncalcified filaments of the alga. The segments with two holes at the broader end were no doubt situated at the base of dichotomising branches as shown in fig. 33, K. The restoration of Ovulites, shown in fig. 33, K, bears a striking resemblance to the figure of an Australian Penicillus given by Harvey in his Phycologia Australica[292].

It is probable that these Eocene forms agreed closely in habit with the recent species of Penicillus. The portions preserved as fossils are segments of the filaments which probably formed a terminal brush of fine branches supported on a stem. The retention of the original generic name Ovulites is on the whole better than the inclusion of the fossil species in the recent genus. The Tertiary species lived in warm seas of the Lower and Middle Eocene of England, Belgium, France and Italy.

Halimeda.

An example of an Eocene species of Halimeda has been recorded by Fuchs from Greifenstein under the name of Halimeda Saportae[293]. The impression has the form of a branched plant consisting of wedge-shaped or oval segments, and there is a close resemblance to the thallus of a recent Halimeda, e.g. H. gracilis Harv. It is not improbable that Fuchs’ determination is correct, but without more definite evidence than is afforded by a mere impression it is a little rash to make use of the recent generic name.

γ. Dasycladaceae.

In this family of Siphoneae are included a number of genera represented by species living in tropical and subtropical seas.

The thallus consists of an elongated axial cell fixed to the substratum by basal rhizoids, and bearing whorls of lateral appendages of limited growth which may be either simple or branched. Many of the lateral branches bear sporangia or spores. The thallus is in many species encrusted with carbonate of lime.

The two genera Acetabularia and Cymopolia may be briefly described as recent types which are represented by trustworthy fossil forms.

Fig. 34. Acetabularia mediterranea Lamx. From a specimen in the Cambridge Botanical Museum (nat. size).

Acetabularia. Figs. 33, I, and 34.

With the exception of A. mediterranea Lamx. (fig. 34) the few living species of this genus are confined to tropical seas.

The habit of Acetabularia is well illustrated by the photograph of a cluster of plants of A. mediterranea Lamx.[294] reproduced in fig. 34. The thallus consists of a delicate stalk attached to the substratum by a tuft of basal holdfasts, and expanded distally into a small circular disc 10–12 mm. in diameter and more or less concave above. This terminal cap is made up of a number of laterally fused appendages given off from the upper part of the stalk in the form of a crowded whorl. The whole thallus resembles a small and long-stalked calcareous fungus. In each radially elongated compartment of the fertile cap (fig. 33, I) there are several sporangia (gametangia) developed; these eventually open and produce numerous ciliated gametes which give rise to zygospores by conjugation. Fig. 33, I, represents the cap of an Acetabularia in radial section and surface-view; the two radial compartments seen in section contain the elliptical gametangia; the circular markings at the base of the figure are scars of sterile deciduous branches.

The whole plant is unicellular, each chamber in the disc being in open communication with the stem of the plant.

Acicularia. Fig. 33, C–H.

In a recent monograph on the Acetabularieae, Solms-Laubach[295] has described a new type of these algae which is of special importance from the point of view of the past history of the family. Möbius described an example of Acetabularia in 1889 under the name A. Schencki; this species has since been placed in D’Archiac’s genus Acicularia[296]. Acicularia Schencki[297] bears a close resemblance as regards external form to Acetabularia mediterranea. In the latter species the walls of the terminal disc compartments are calcified, and the cavity of each of the laterally fused members contains numerous free spores; in Acicularia, the cavity of each disc-ray is occupied by a calcareous substance in the form of a spicule containing numerous cavities in each of which is a single sporangium. A single spicule is seen in fig. 33, H, showing the spherical pockets in which the sporangia were originally situated. This species, Acicularia Schencki, has been recorded from Martinique, Guadeloupe, Brazil, and a few other places.

The genus Acicularia was founded by D’Archiac for certain minute calcareous spicules found in the Eocene sands (Calcaire Grossier) of the Paris basin. D’Archiac describes one species, Acicularia pavantina, which he defines as follows:—“Polypier aciculaire, élargi, et légèrement comprimé à sa partie supérieure, qui est échancrée au milieu. Surface couverte de petits pores simples, nombreux, disposés irrégulièrement[298].” The same species is figured also in Michelin’s Iconographie Zoophytologique, and described as an organism of which the exact zoological position is uncertain[299]. After these fossils had been placed in various divisions of the animal kingdom, Carpenter[300] described several specimens as portions of foraminifera. Finally, Munier-Chalmas removed Acicularia to the plant kingdom, and “with rare divination” placed the genus among the Acetabularieae. The history of our knowledge of the true nature of Acicularia is of unusual interest. Some of the specimens of this genus figured in Carpenter’s monograph have the form of imperfect long and narrow bodies tapering to a point at one end and broad at the other (fig. 33, F and G); they are joined together laterally and pitted with numerous small cavities. From the resemblance of such specimens to a fragment of the terminal fertile disc of the recent Acetabularias, Munier-Chalmas referred the fossils to this type of algae. In the living species which were then known the radiating chambers of the disc contained loose sporangia, without any calcareous matrix filling the cavity of the chambers. In the fossil Acicularias, on the other hand, the manner of preservation of the pitted calcareous spicules pointed to the occurrence of sporangia embedded in cavities in a calcareous matrix. Subsequent to Munier-Chalmas’ somewhat daring conclusions as to the relation of Acicularia to Acetabularia, Solms-Laubach found that the species originally described by Möbius as Acetabularia Schencki from Guadeloupe presented exactly those characters in which the fossil specimens differ from Acetabularia. The genus Acicularia formerly restricted to fossil species is now applied also to this single living species Acicularia Schencki.

The genus is thus defined by Solms-Laubach:—

“Discus fertilis terminalis e radiis inter se conjunctis formatus, coronis et inferiore et superiore praeditis, sporae massa mucosa calce incrustata coalitae, pro radio spiculam solidam cuneatam formantes[301].”

As Solms-Laubach points out in his recent monograph, Munier-Chalmas’ conjecture, “which had little to support it in the fossil material, has been more recently proved true in the most brilliant fashion by the discovery of a living species of this genus.”

•••••

1. Acicularia Andrussowi Solms[302]. Fig. 33, C and D. This species was first described by Andrussow[303] as Acetabularia miocenica from the Crimea. It occurs in Miocene rocks south of Sevastopol, and, with Ostrea and Pecten, forms masses of white limestone.

In each sporangial ray of the disc the cavity contains a calcareous spicula bearing spore cavities in four rows. “Round each spore-cavity there is a circular zone which stands out, when viewed in reflected light, through its white colour against the central mass of the spicule, though a sharp contour is not visible[304].” Fig. 33, C, is taken from a somewhat diagrammatic sketch by Andrussow; it shows ten of the fertile rays of the disc. The thick walls of the chambers are seen in the two lowest rays, and in the next two rays the spore-cavities are represented. A more accurate drawing, from Solms-Laubach’s memoir, is reproduced in fig. 33, D. The calcareous spicule with numerous spore-cavities shown in fig. 33, H, is from a fertile ray of the recent species Acicularia Schencki. This corresponds to the spore-containing calcareous matrix in each ray of the disc of Acicularia Andrussowi Solms. The spicule copied in fig. 33, F from one of Carpenter’s drawings[305] of an Eocene specimen bears the closest resemblance to the recent spicule of fig. 33, H, and emphasizes the very close relationship between the fossil forms and the single rare tropical species.

2. Acicularia miocenica Reuss. Another Tertiary species has been described under this name by Reuss[306] from the Miocene of the Vienna district, from the Leithakalk of Moravia and elsewhere. It agrees very closely with the recent species A. Schencki. A section of one of the spicules of this species is shown in fig. 33, E; the dark patches represent the pockets in the calcareous spicule which were originally occupied by sporangia and spores.

Cymopolia. Fig. 33, A, B, M and N.

The genus Cymopolia is at present represented by two species, C. barbata (L.) and C. mexicana, Ag., living in the Gulf of Mexico and off the Canary Islands.

Cymopolia and Acetabularia, with several other calcareous algae, are figured by Ellis and other writers as members of the animal kingdom. Ellis speaks of the species of Cymopolia which he figures as the Rosary Bead-Coralline of Jamaica.

Fig. 33, M, has been drawn from a figure published by Ellis in his Natural History of the Corallines published in 1755[307]. The thallus has the form of a repeatedly forked body, of which the branches are divided into cylindrical joints thickly encrusted with carbonate of lime, but constricted and uncalcified at the limits of each segment. A tuft of hairs is given off from the terminal segment of each branch. The axis of each branch of the thallus is occupied by a cylindrical and unseptate cell which gives off crowded whorls of lateral branches. In the lower part of fig. 33, M, the calcareous investment has been removed, and the branches are seen as fine hair-like appendages of the central cell. The branches given off from the constricted portions of the axis are unbranched simple appendages, but the others terminate in bladder-like swellings, each of which bears an apical sporangium. The sporangia are surrounded and enclosed by the swollen tips of four to six branches which spring from the summit of the sporangial branch. Fig. 33, A, represents part of a transverse section through the calcareous outer portion of a branch of Cymopolia; the darker portions or cavities in the calcareous matrix were originally occupied by the lateral branches and sporangia[308].

In Fig. 33, B, the sporangial branch with the terminal sporangium and three of the investing branches are more clearly shown, the surrounding calcareous investment and the thallus having been removed by the action of an acid.

In a transverse section of a branch from which the organic matter had been removed, and only the calcareous matrix left, one would see a central circular cavity surrounded by a thick calcareous wall perforated by radially disposed canals and containing globular cavities; the canals and cavities being occupied in the living plant by branches and sporangia respectively.

The two circular cavities shown in the figure mark the position of the sporangia which are borne on branches with somewhat swollen tips. From the summit the left-hand sporangial branch shown in fig. 33, A, three of the secondary branches are represented by channels in the calcareous matrix; the two black dots on the face of the sporangiophore being the scars of the remaining two secondary branches.

By the lateral contact of the swollen ends of the ultimate branches enclosing the sporangia the whole surface of the thallus, when examined with a lens, presents a pitted appearance. Each pit or circular depression (fig. 33, N) marks the position of the swollen tip of a branch.

This form of thallus represents a type which is met with in several members of the Dasycladaceae. It would carry us beyond the limits of a short account to describe additional recent genera which throw light on the numerous fossil species. For further information as to the recent members of the family, the student should refer to Murray’s Seaweeds[309], and for a more detailed memoir on the group to Wille’s recent contribution to the Pflanzenfamilien[310] of Engler and Prantl. Among the various special contributions to our knowledge of the Dasycladaceae, those by Munier-Chalmas[311], Cramer[312], Solms-Laubach[313], and Church[314], may be mentioned.

PALAEOZOIC SIPHONEAE.

The publication of a short preliminary note by Prof. Munier-Chalmas in the Comptes Rendus for 1877 was the means of calling attention to the exceptional importance of the calcareous Siphoneae as algae possessing an interesting past history, of which satisfactory records had been preserved in rocks of various ages. Decaisne had pointed out in 1842 that certain marine organisms previously regarded as animals should be transferred to the plant kingdom. Such seaweeds as Halimeda, Udotea, Penicillus and others were thus assigned to their correct position. Many fossil algae belonging to this group continued to be dealt with as Foraminifera until Munier-Chalmas demonstrated their true affinities. In Gümbel’s monograph on the so-called Nullipores found in limestone rocks, published in 1871[315], several examples of siphoneous algae are included among the fossil Protozoa.

In recent years there have been several additions to an already long list of fossil Siphoneae. In addition to the numerous and well-preserved specimens, representing a large number of generic and specific forms, which have been collected from the Eocene of the Paris basin, there is plenty of evidence of the abundance of the members of the Dasycladaceae in the Triassic seas. In the Triassic limestones of the Tyrol, as well as in other regions, the calcareous bodies of siphoneous algae have played no inconsiderable part as agents of rock-building[316]. Genera have been recorded from Silurian and other Palaeozoic horizons, and there is no doubt that the Verticillate Siphoneae of to-day are the remnants of an extremely ancient family, which in former periods was represented by a much more widely distributed and more varied assemblage of species. There is probably no more promising field of work in the domain of fossil algae than the further investigation of the numerous forms included in Munier-Chalmas’ class of Siphoneae Verticillatae. A brief description of a few genera from different geological horizons must suffice to draw attention to the character of the data for a phylogenetic history of this group.

The fossil examples of the genus Cymopolia (Polytrypa) were originally described by Defrance[317] in the Dictionnaire des Sciences Naturelles as small polyps under the generic name Polytrypa.

In the Eocene sands of the Paris basin there have been found numerous specimens of short, calcareous tubes which Munier-Chalmas has shewn are no doubt the isolated segments of an alga practically identical with the recent Cymopolia. A section[318] through one of the fossil segments presents precisely the same features as those which are represented in fig. 33, A. The habit of the Eocene alga and its minute structure were apparently almost identical with those of the recent species, Cymopolia barbata. The two drawings of Cymopolia reproduced in fig. 33, A and B, have been copied from Munier-Chalmas’ note in the Comptes Rendus[319]; the corresponding figures given by this author of the Eocene species (Cymopolia elongata Deb.) are practically identical with figs. A and B, and show no points of real difference. The segments of the thallus of the fossil species, as figured by Defrance[320], appear to be rather longer than those of the recent species. The calcareous investment of the axial cell of the thallus was traversed by regular verticils of branches or ‘leaves’; the central branch of each whorl terminates in an oval sporangial cavity, exactly as in fig. 33, A and B; and from the top of this branch there is given off a ring of slender prolongations which terminate on the surface of the calcareous tube as regularly disposed depressions, which were no doubt originally occupied by their swollen distal ends as in the recent species.

Vermiporella.

This generic name was proposed by Stolley for certain branched and curved tubes found in Silurian boulders from the North German drift[321]. The tubes have a diameter of ·5–1 mm., and are perforated by radial canals which probably mark the position of verticils of branches given off at right angles to the central axis. The surface of the tubes is divided into regular hexagonal areas.

The resemblance of these Silurian fossils to Diplopora and other genera favours their inclusion in the Verticillate Siphoneae.

Sycidium. Fig. 32, B.

The fossils included in this genus were first described by Sandberger from the middle Devonian rocks of the Eifel, and referred by him to the animal kingdom. More recently Deecke has suggested the removal of the genus to the calcareous Siphoneae, and such a view appears perfectly reasonable, although without more data it is not possible to speak with absolute certainty.

Sycidium melo. (Sandb.) Fig. 32, B. The specimen represented in fig. 32, B (i), (ii), drawn from Deecke’s figures[322], has the form of a small oval calcareous body, 1 mm. in transverse diameter and 1–1·3 mm. in longitudinal diameter. It is pointed at one end and flattened at the other. At the flatter end there is a circular depression, continued into a funnel-shaped cavity, and on the walls of this cavity there are 18–20 radially disposed ribs, which extend over the surface of the whole body. A series of transverse ribs intersects the vertical ribs at right angles. The calcareous wall is perforated by numerous whorls of circular pores, and the internal cavity is a simple undivided space. Each of these oval bodies (fig. 33, B) is probably the segment of a thallus, and the perforations in the wall may have been originally occupied by lateral prolongations from the unseptate axial cell of the thallus. Sycidium bears a fairly close resemblance to the Tertiary Ovulites.

Diplopora. Fig. 35, A and B.

This genus of algae is characteristic of Triassic rocks, and is especially abundant in Muschelkalk and Lower Keuper limestones of the Alps, Silesia, and elsewhere. The thallus, or rather the calcareous portion of the thallus, has the form of a thick-walled tube, with a diameter of about 4 mm., and occasionally reaching a length of 50 mm. At one end the tube has a rounded and closed termination, and the wall is pierced throughout its whole length by regular whorls of fine canals. Diplopora agrees with Cymopolia in its main features.

Fig. 35. A, B, Diplopora. × 2. C, D, Gyroporella (after Benecke. × 4). E, Calcareous segments of Penicillus, from a specimen in the British Museum. × 5. F, a single segment of Ovulites margaritula Lam. × 4. G, Confervites chantransioides Born. (after Bornemann. × 150).

Fig. 35, A, affords a diagrammatic view of a Diplopora tube, and shews the arrangement of the numerous whorls of canals. In fig. 35, B, a piece of limestone is represented containing several Diploporas cut across transversely and more or less obliquely. In an obliquely transverse section of a tube perforated by horizontal canals the cavities of the canals necessarily appear as holes or discontinuous canals in the substance of the calcareous wall. The manner of occurrence of the specimens points to the abundance of this genus in the Triassic seas, and suggests that the calcareous tubes of Diplopora may have been important factors in the building up of limestone sediments[323]. In many instances no doubt the carbonate of lime of the thallus has been dissolved and recrystallised, and the original form completely obliterated. As in the rocks built up largely of calcareous Florideae (p. 185) which have lost their structure, it is a legitimate inference that some of the limestone rocks which shew no trace of organic structure may have been in part derived from the calcareous incrustation of various algal genera.

Gyroporella. Fig. 35, C and D.

In this genus from the Alpine Trias the structure of the calcareous tube is very similar to that in Diplopora, but in Gyroporella the canals form less distinct whorls and are closed externally by a small plate, as seen in figs. 35, C and D.

As Solms-Laubach has pointed out, the branch-systems of Diplopora, Gyroporella and other older genera are much simpler than in the Tertiary genera Dactylopora and others[324].

A species of Gyroporella, G. bellerophontis, has recently been described by Rothpletz[325] from Permian rocks in the Southern Tyrol. The thallus is tubular in form and has a diameter of ·5–1 mm.

Dactylopora.

The genus Dactylopora was founded by Lamarck[326] on some fossil specimens from the Calcaire Grossier and included among the Zoophytes. D’Orbigny afterwards included it among the Foraminifera, and the structure of the calcareous body has been described by Carpenter[327] and other writers on the Foraminifera. In a specimen of Dactylopora cylindracea Lam. from the Paris basin, for which I am indebted to Munier-Chalmas, the tubular thallus measures 4 mm. in diameter; at the complete end it is closed and bluntly rounded. The wall of the tube is perforated by numerous canals, and contains oval cavities which were no doubt originally occupied by sporangia. The shape of the specimens is similar to that of Diplopora, but the canals and cavities present a characteristic and more complex appearance, when seen in a transverse section of the wall, than in the older genus Diplopora. Gümbel has given a detailed account of this Tertiary genus in his memoir on Die sogenannten Nulliporen[328]; he distinguishes between Dactyloporella and Gyroporella by the existence of cavities in the calcareous wall of the tube in the former genus, and by their absence in the latter. The oval cavities in a Dactyloporella were originally occupied by sporangia; in Diplopora and Gyroporella the sporangia were probably borne externally and on an uncalcified portion of the thallus.

•••••

In addition to the few examples of fossil species described above there are numerous others of considerable interest, which illustrate the great wealth of form among the Tertiary and other representatives of the Verticillate Siphoneae.

Reference has already been made to Vermiporella as an example of a Silurian genus. Other genera have been described by Stolley from Silurian boulders in the North-German drift under the names Palaeoporella, Dasyporella and Rhabdoporella[329]; the latter genus is compared with the Triassic Diplopora, and the two preceding with the recent Bornetella.

Schlüter has transferred a supposed Devonian Foraminiferal genus, Coelotrochium[330], to the list of Palaeozoic Siphoneae. Munier-Chalmas regards some of the fossils described by Saporta under the name of Goniolina[331], and classed among the inflorescences of pro-angiospermous plants, as examples of Jurassic Siphoneae. The shape and surface-features of some of the examples of Goniolina suggest a comparison with Echinoid spines, but the resemblance which many of the forms in the Sorbonne collection present to large calcareous Siphoneae is still more striking. A comparison of Saporta’s fig. 5, Pl. xxxiii. and fig. 4, Pl. xxxii. in volume iv. of the Flore Jurassique, with the figures given by Solms-Laubach[332] and Cramer[333] of species of Bornetella brings out a close similarity between Goniolina and recent algae; the chief difference being the greater size of the fossil forms. The possibility of confounding Echinoid spines with calcareous Siphoneae is illustrated by Rothpletz[334], who has expressed the opinion that Gümbel’s Haploporella fasciculata is not an alga but the spine of a sea-urchin.

Among Cretaceous forms, in addition to Goniolina, which passes upwards from Jurassic rocks, Triploporella[335] and other genera have been recorded.

Uteria[336] is an interesting type of Tertiary genera; it occurs in the form of barrel-shaped rings, which are probably the detached segments of a form in which the central axial cell was encrusted with carbonate of lime, but the sporangia and the whorls of branches differed from those of Cymopolia in being without a calcareous investment.

b. Confervoideae.

Without attempting to describe at length the fossil forms referred to this division of the Chlorophyceae, there is one fossil which deserves a passing notice. Brongniart in 1828[337] instituted the generic term Confervites for filamentous fossils resembling recent species of confervoid algae. Numerous fossils have been referred to this genus by different authors, but they are for the most part valueless and need not be further considered. In 1887 Bornemann described some new forms which he referred to this genus from the Cambrian rocks of Sardinia. He describes the red marble of San Pietra, near Masne, as being in places full of the delicate remains of algae having the form of branched filaments, and appearing in sections of the rock as white lines on a dark crystalline matrix. In fig. 35, G, one of these Sardinian specimens is represented. This form is named Confervites Chantransioides[338]; the thallus consists of branched cell-filaments, having a breadth of 6–7µ, and composed of ovate cells. It is possible that this is a fragment of a Cambrian alga, but the figures and descriptions do not afford by any means convincing evidence. From post-Tertiary beds various genera, such as Vaucheria and others, have been recorded, but they possess but little botanical value.

C. INCERTAE SEDIS.

Fossils in Boghead ‘Coal’ referred by some authors to the Chlorophyceae.

During the last few years much has been written by two French authors, Dr Renault and Prof. Bertrand, on the subject of the so-called Boghead of France, Scotland, and other countries. They hold the view that the formation of the extensive beds of this carbonaceous material was due to the accumulation and preservation of enormous numbers of minute algae which lived in Permo-Carboniferous lakes.

In an article contributed to Science-Progress in 1895 I ventured to express doubts as to the correctness of the conclusions of MM. Renault and Bertrand[339]. Since then Prof. Bertrand has very kindly demonstrated to me many of his microscopic preparations of various Bogheads, and I am indebted to Prof. Bayley Balfour of Edinburgh for an opportunity of examining a series of sections of the Scotch Boghead. The examination of these specimens has convinced me of the difficulties of the problems which many investigators have tried to solve, but it has by no means led me to entirely adopt the views expressed by MM. Bertrand and Renault.

BOGHEAD.

The Boghead or Torbanite of Scotland was rendered famous by a protracted lawsuit tried in Edinburgh from July 29th to August 4th, 1853. A lease had been granted by Mr and Mrs Gillespie, of Torbanehill, in Fifeshire, to Messrs James Russell and Son, coal-masters of Falkirk, of “the whole coal, ironstone, iron-ore, limestone, and fire-clay (but not to comprehend copper, or any other minerals whatsoever, except those specified) with lands of Torbanehill[340].” After the Boghead had been worked for two years the Gillespies challenged the right of Messrs Russell, and argued that the valuable mineral Torbanite was not included among the substances named in the agreement. The defendants maintained that it was a coal, known as gas-, cannel- or parrot-coal. A verdict was given for the defendants. Some of the scientific experts who gave evidence at the trial considered that the Boghead afforded indications of organic structure, while others regarded it as essentially mineral in origin.

The Torbanite or Boghead is a close-grained brown rock, of peculiar toughness and having a subconchoidal fracture. It contains about 65% carbon, with some hydrogen, oxygen, sulphur, and mineral substances. A thin section examined under the microscope presents the appearance of a dark and amorphous matrix, containing numerous oval, spherical and irregularly shaped bright orange-yellow patches. Fig. 36, 1 shows the manner of occurrence of the yellow bodies in a piece of Scotch Boghead, as seen in a slightly magnified horizontal section. Under a higher power the light patches in the figure reveal traces of a faint radial striation, which in some cases suggests the occurrence of a number of oval or polygonal cells.

The Autun Boghead possesses practically the same structure. The yellow bodies are often sufficiently abundant to impart a bright yellow colour to a thin section. If the section is vertical the coloured bodies are seen to be arranged in more or less regular layers parallel to the plane of bedding.

The Kerosene shale of New South Wales agrees closely with the Scotch and French Boghead; it is approximately of the same geological age, and is largely made up of orange or yellow bodies similar to those of the European Boghead, but much more clearly preserved.

The nature and manner of formation of the various forms of coal should be dealt with in a later chapter devoted to the subject of plants as rock-builders, but in view of the recent statements as to the algal nature of these bituminous deposits it may not be out of place to state briefly the main conclusions of the French authors.

MM. Renault and Bertrand regard each of the yellow bodies in the European and Australian Boghead as the thallus of an alga. To the form which is most abundant in the Kerosene shale they have given the generic name of Reinschia, while that in the Scotch and French Boghead is named Pila.

Reinschia. Fig. 36, 3.

A section of a piece of Kerosene shale at right angles to the bedding appears to be made up of fairly regular layers of flattened elliptical sacs of an orange or yellow colour. Each sac or thallus is about 300µ in length and 150µ broad (fig. 36, 3). A single row of cells constitutes the wall surrounding the central globular cavity. The cells are more or less pyriform in shape, and the cell-cavities are filled with a dark substance, described by Renault and Bertrand as protoplasm, and the cell-walls are fairly thick. In some of the larger specimens there are often found a few smaller sacs enclosed in the cavity of the partially disorganised mother-thallus. In the larger specimens the wall is usually invaginated in several places, giving the whole thallus a lobed or brain-like appearance. The supposed alga, which makes up ⁹⁄₁₀ths of the contents of a block of Kerosene shale, is named Reinschia Australis; it is regarded by the authors of the species as nearly related to the Hydrodictyaceae or Volvocineae.

Fig. 36, 1. Section of a piece of Scotch Torbanite. Slightly enlarged. 2. Pila bibractensis from the Autun Boghead, × 282 (after Bertrand). 3. Reinschia Australis, from the Kerosene shale of New South Wales, × 592 (after Bertrand).

In the Kerosene shale from certain localities in New South Wales Bertrand recognises a second form of thallus, which he refers to the genus Pila, characteristic of the European Bogheads.

Pila. Fig. 36, 2.

The “thallus” characteristic of the Scotch Boghead has been named Pila scotica, and that of the Autun Boghead, Pila bibractensis.

In the latter form, which has been studied in more detail by MM. Renault and Bertrand, the thallus consists of about 6–700 cells, and is irregularly ellipsoidal in form, from ·189–·225mm. in length, and ·136–·160mm. broad. The surface-cells are radially disposed and pyramidal in shape, the internal cells are polygonal in outline and less regularly arranged (fig. 36, 2). The Pila thalli make up ¾ths of the mass in an average sample of the Autun Boghead. The Autun Boghead often contains siliceous nodules, and sections of these occasionally include cells of a Pila in which the protoplasmic contents and nuclei have been described by the French authors. The evidence for the existence of these supposed nuclei is, however, not entirely satisfactory; sections of silicified thalli which were shown to me by Prof. Bertrand did not satisfy me as to the minute histological details recognised by Bertrand and Renault.

The species of Pila are compared with the recent genus Celastrum, and regarded as most nearly allied to the Chroococcaceae or Pleurococcaceae among recent algae. Prof. Bornet[341] has suggested Gomphosphaeria as a genus which presents a resemblance to the Autun Pila.

In addition to the Bogheads of Autun, Torbanehill, and New South Wales, there are similar Palaeozoic deposits in Russia, America, and various other parts of the world. Full details of the structure of Boghead and the supposed algae referred to Reinschia, Pila, and other genera will be found in the writings of Bertrand and Renault[342].

The Kerosene shale of New South Wales affords the most striking and well-preserved examples of the cellular orange and yellow bodies referred to as the globular thalli of algae. It is almost impossible to conceive a purely inorganic material assuming such forms as those which occur in the Australian Boghead. On the other hand, it is hardly less easy to understand the possibility of such explanations as have been suggested of the organic origin of these characteristic bodies.

The ground-mass or matrix of the Boghead is referred to a brown ulmic precipitate thrown down on the floor of a Permian or Carboniferous lake, probably under the action of calcareous water. In this material there accumulated countless thalli of minute gelatinous algae, which probably at certain seasons completely covered the surface of the waters, as the fleurs d’eau in many of our fresh-water lakes. In addition to the thalli of Reinschia and Pila the Bogheads contain a few remains of various plant fragments, pollen-grains, and pieces of wood. Fish-scales and the coprolites of reptiles and fishes occur in some of the beds. On a piece of Kerosene shale in the Woodwardian Museum, Cambridge, there are two well-preserved graphitic impressions of the tongue-shaped fronds of Glossopteris Browniana, Brongn. There can be little doubt that the beds of Boghead were deposited under water as members of a regular sequence of sedimentary strata. The yellow bodies which form so great a part of the beds are practically all of the same type. Reinschia and Pila cannot always be distinguished, and it would seem that there are no adequate grounds for instituting two distinct genera and referring them to different families of recent algae.

Stated briefly, my conclusion is that the algae of the French authors may be definite organic bodies, but it is unwise to attempt to determine their affinities within such narrow limits as have been referred to in the above résumé. The structure of the bituminous deposits is worthy of careful study, and it is by no means impossible that further research might lead us to accept the view of the earlier investigators, that the brightly coloured organic-like bodies may be inorganic in origin.

D. RHODOPHYCEAE. (Florideae. Red Algae.)

The thallus of the members of this group assumes various forms, and consists of branched cell-filaments of a more or less complex structure. Cells of the thallus contain a red colouring matter in addition to the green chlorophyll. The reproduction is asexual and sexual; the formation of asexual reproductive cells (tetraspores) in groups of four in sporangia is a characteristic method of reproduction. Sexual reproduction is effected by means of distinct male and female cells.

With the exception of a few fresh-water genera all the red algae are marine. The Rhodophyceae, like the Cyanophyceae and Chlorophyceae, include a shell-boring form which has been found in the common razor-shell[343]. Several genera live as endophytes in the tissues of other algae. The recent species of this section of algae are characteristic of temperate and tropical seas. One subdivision of the red algae, the Corallinaceae, is extremely important from a geological point of view and must be dealt with in some detail.

Corallinaceae.

The thallus is usually encrusted with carbonate of lime; it is of a branched cylindrical form in the well-known Corallina officinalis, Linn. of the British coasts, of an encrusting and foliaceous type, in the genus Lithophyllum, and of a more coral-like form in the genus Lithothamnion. The reproductive organs occur in conceptacles, having the form of small depressed cavities in the thallus, or projecting as warty swellings above the surface of the plant. Asexual reproduction is by means of tetraspores formed in conceptacles resembling those containing the sexual cells. The Corallinaceae may be subdivided into the two families Melobesieae and Corallineae[344].

Melobesieae.	Thallus encrusting, leaf- or coral-like; unsegmented. (Melobesia, Lithophyllum, Lithothamnion.)
Corallineae.	Cylindrical filamentous and segmented thallus. (Amphiroa and Corallina.)

The genus Corallina is the best known British representative of the Corallinaceae. With other members of the group it was long regarded as a coralline animal, and it is only comparatively recently that the plant-nature of these forms has been generally admitted. Lithophyllum, Lithothamnion, Melobesia, and other genera of the Corallinaceae and some of the Siphoneae play a very important part in the building and cementing of coral-reefs. The pink or rose-coloured calcareous thallus of some of these calcareous algae or Nullipores imparts to coral-reefs a characteristic appearance. In some cases, indeed, the coral-reefs are very largely composed of algae. Saville Kent[345] describes the Corallines or Nullipores of the Australian Barrier-reef as furnishing a considerable quota towards the composition of the coral rock. Mr Stanley Gardiner, who accompanied the coral-boring expedition to the island of Funafuti, has kindly allowed me to quote the following extract from his notes, which affords an interesting example of the importance of calcareous algae as reef-building organisms. “It is quite a misnomer to speak of the outer edge of a reef like this (Rotuma Island) as being formed of coral. It would be far better to call it a Nullipore reef, as it is completely encrusted by these algae, while outside in the perfectly clear water, 10 to 15 fathoms in depth, the bottom has a most brilliant appearance from masses of red, white and pink Nullipores, with only a stray coral here and there.”

Agassiz[346] has given an account of the occurrence of immense masses of Nullipores (Udotea, Halimeda etc.) in the Florida reefs; his description is illustrated by good figures of these algae.

In the Mediterranean there are true Nullipore reefs, which are interesting geologically as well as botanically. Walther[347] has described one of these limestone-banks in the Gulf of Naples which occurs about 1 kilometre from the coast and 30 metres below the surface of the water. Every dredging, he says, brings up numberless masses of Lithothamnion fasciculatum (Lamarck), and L. crassum (Phil.). Between the branches of the algae, gasteropods and other animals become completely enclosed by the growing plants, while diatoms, foraminifera, and other forms of life are abundant. Water percolating through the mass gradually destroys the structure of the algal thalli, and in places reduces the whole bank to a compact structureless limestone.

The same author[348] has also called attention to the importance of Lithophyllum as a constructive element in the coral-reefs off the Sinai peninsula.

Lithothamnion a typical genus of the Corallinaceae may be briefly described.

Lithothamnion. Fig. 37.

Philippi[349] was the first writer to describe this and other genera as plants. He gave the following definition of Lithothamnion:

“Stirps calcarea rigida, e ramis cylindricis vel compressiusculis dichotoma ramosis constans.”

The thallus of Lithothamnion grows attached to the face of a rock or other foundation, and forms a hard, stony mass, assuming various coralline shapes. The exposed face may have the form of numerous short branches or of an irregular warty surface.

In section (fig. 37, A.) the lower part of the thallus is seen to be made up of rows of cells radiating out from a central point, and the upper portion consists of vertical and horizontal rows of cells. The whole body is divided up into a large number of small cells by anticlinal and periclinal walls, and possesses an evident cellular as distinct from a tubular structure. Conceptacles containing reproductive organs are either sunk in the thallus or project above the surface. The two types of structure in a single thallus are shown in fig. 37, A, also a conceptacle containing tetraspores.

Fig. 37, A. Section of a recent Lithothamnion (after Rosanoff[350], × 200). B. Section of Lithothamnion suganum, Roth (after Rothpletz[351], × 100). C. A conceptacle with tetraspores from a Tertiary Lithothamnion (after Früh[352], × 300). D. Sphaerocodium Bornemanni Roth, (after Rothpletz, × 150).

In the closely allied Lithophyllum the thallus is encrusting, and in section it presents the same appearance as the lower part of a Lithothamnion thallus.

Species of Lithothamnion occur in the Mediterranean Sea, and are abundant in the arctic regions[353], while on the British coasts the genus is represented by four species[354]. Some large specimens of Lithothamnion and Lithophyllum are exhibited in one of the show-cases in the botanical department of the British Museum. For the best figures and descriptions of recent species reference should be made to the works of Hauck, Rosanoff, Rosenvinge, Kjellman and Solms-Laubach[355].

It is to be expected that such calcareous algae as Lithothamnion should be widely represented by fossil forms. In addition to the botanical importance of the data furnished by the fossil species as to the past history of the Corallinaceae, there is much of geological interest to be learnt from a study of the manner of occurrence of both the fossil and recent representatives. As agents of rock-building the coralline algae are especially important. The late Prof. Unger[356] in 1858 gave an account of the so-called Leithakalk of the Tertiary Vienna basin, and recognised the importance of fossil algae as rock-forming organisms. The Miocene Leithakalk, which is widely used in Vienna as a building stone[357], consists in part of limestone rocks consisting to a large extent of Lithothamnion.

Since the publication of Unger’s work several writers have described numerous fossil species of Lithothamnion from various geological horizons. A few examples will suffice to illustrate the range and structure of this and other genera of the Corallinaceae. In dealing with the fossil species it is often impossible to make use of those characters which are of primary importance in the recognition of recent species. The fossil thallus is usually too intimately associated with the surrounding rock to admit of any use being made of external form as a diagnostic feature. The size and form of the cells must be taken as the chief basis on which to determine specific differences. In the absence of conceptacles or reproductive organs it is not always easy to distinguish calcareous algae from fossil Hydrozoa or Bryozoa. In many instances, however, apart from the nature and size of the elements composing the thallus, the conceptacles afford a valuable aid to identification. An example of a fossil conceptacle containing tetraspores is shown in fig. 37, C; it is from a Tertiary species of Lithothamnion, described by Früh from Montévraz in Switzerland.

•••••

1. Lithothamnion mamillosum Gümb. Fig. 32, A (i) and (ii). (p. 155.) This species was first recorded by Gümbel[358] from the Upper Cretaceous (Danian) rocks of Petersbergs, near Maëstricht, on the Belgian frontier. It was originally described as a Bryozoan. The thallus has the form of an encrusting calcareous structure bearing on its upper surface thick nodular branches, as shown in fig. 32, A (ii); in section, A (i), the thallus consists of a regular series of rectangular cells.

The specific name mamillosum has also been given to a recent species by Hauck[359], but probably in ignorance of the existence of Gümbel’s Cretaceous species.

•••••

2. Lithothamnion suganum Roth. Fig. 37, B. The section of this form given in fig. 37, B shows three oval conceptacles filled with crystalline material. The two lower conceptacles originally communicated with the surface of the thallus, but as in recent species the deeper portions of the algal body became covered over by additions to the surface, forming merely dead foundations for new and overlying living tissues.

The cells of the thallus have a breadth of 7–9µ, and a length of 9–12µ.

The specimen was obtained from a Lithothamnion bank, probably of Upper Oligocene age, in Val Sugana[360], in the Austrian Tyrol.

Numerous other species of Jurassic, Cretaceous and Tertiary age might be quoted, but the above may suffice to illustrate the general characters and mode of occurrence of the genus. It is important that the student should become familiar with the Lithothamnion and Lithophyllum types of thallus, in view of their frequent occurrence in crystalline limestone rocks and in such comparatively recent deposits as those of upraised coral-reefs. The coral-rock of Barbados and other West-Indian islands affords a good illustration of the manner of occurrence of fossil coralline algae in association with corals and other organisms[361].

In the fossil species of Lithothamnion hitherto recorded there do not appear to be any important features in which they differ from recent forms; the geological history of the genus so far as it is known, favours the view that the generic characters are of considerable antiquity.

Solenopora. Fig. 38.

Mr A. Brown[362], of Aberdeen, has recently brought forward good evidence for including various calcareous fossils, described by several authors under different names and referred to various genera of fossil animals, in the genus Solenopora, which he places among the coralline algae.

Species of this genus have been described from England, Scotland, Esthonia, Russia, and other countries. The geological range of Solenopora appears to be from Ordovician to Jurassic rocks; in some cases it is an important constituent of beds of limestone.

•••••

Solenopora compacta (Billings). Fig. 38. This species was originally described by Billings as Stromatopora compacta, and afterwards defined by Nicholson and Etheridge. The thallus forms sub-spheroidal masses, from the size of a hemp-seed to that of an orange. The external surface is lobulate; the fractured surface has a porcellaneous and sometimes a fibrous appearance, and is usually white or light brown in colour. In vertical section (fig. 38, B) the cells are elongated and arranged in a radiating and parallel fashion; they often occur in concentric layers. The cells have a diameter of about ¹⁄₁₇ mm. and possess distinctly undulating walls, as seen in a tangential section (fig. 38, A). Brown describes certain larger cells in the thallus (fig. 38, A) as sporangia[363], but it is difficult to recognise any distinct sporangial cavities in the drawing. The example figured is from the Trenton limestone of Canada; a variety of the same species has been recorded from the Ordovician rocks of Girvan in Ayrshire. There appear to be good reasons for accepting Brown’s conclusion that Solenopora belongs to the Corallinaceae rather than to the Hydrozoa, among which it was originally included. After comparing Solenopora with recent genera of Florideae, Brown concludes that “the forms of the cells and cell-walls, the method of increase, and the arrangement of the tissue cells in the various species of Solenopora bear strong evidence of relationship between that genus and the calcareous algae[364].”

Fig. 38. Solenopora compacta (Billings). A. Tangential section. × 100. B. Vertical section. × 50. (After Brown.)

The importance of the calcareous Rhodophyceae has been frequently emphasised by recent researches, and our knowledge of the rock-building forms is already fairly extensive. We possess evidence of the existence of species of different genera in Ordovician seas, as well as in those of the Silurian, Triassic, Jurassic, and more recent periods. It is reasonable to prophesy that further researches into the structure of ancient limestones will considerably extend our knowledge of the geological and botanical history of the Corallinaceae.

Numerous fossils have been described as examples of other genera[365] of Rhodophyceae than those included in the Corallinaceae, but these possess little or no scientific value and need not be considered.

E. PHAEOPHYCEAE (Brown Algae).

Olive-brown algae, thallus often leathery in texture, composed of cell-filaments or parenchymatous tissue, in some cases exhibiting a considerable degree of internal differentiation. The sexual reproductive organs may be either in the form of passive egg-cells and motile antherozoids or of motile cells showing no external sexual difference.

With one or two exceptions all the genera are marine. They have a wide distribution at the present day, and are especially characteristic of far northern and extreme southern latitudes. The gigantic forms Lessonia, Macrocystis and others already alluded to, belong to this group; also the genus Sargassum, of which the numberless floating plants constitute the characteristic vegetation of the Sargasso Sea.

Palaeobotanical literature is full of descriptions of supposed fossil representatives of the brown algae, but only a few of the recorded species possess more than a very doubtful value; most of them are worthless as trustworthy botanical records. Many of the numerous impressions referred to as species of Fucoides and other genera present a superficial resemblance to the thallus of the common Bladder-wrack and other brown seaweeds. Such similarity of form, however, in the case of flat and branched algal-like fossils is of no scientific value. In many instances the impressions are probably those of an alga, but they are of no botanical interest. The flat and forked type of thallus of Fucus, Chondrus crispus (L.) and other members of the Phaeophyceae is met with also among the red and green algae, to say nothing of its occurrence in the group of thalloid Liverworts, or of the almost identical form of various members of the animal kingdom. The variety of form of the thallus in one species is well illustrated by the common Chondrus crispus (L.). This alga was described by Turner[366] in his classic work on the Fuci under the name of Fucus crispus as “a marine Proteus.” It affords an interesting example of the different appearance presented by the same species under different conditions, and at the same time it furnishes another proof of the futility of relying on imperfectly preserved external features as taxonomic characters of primary importance.

An example of a supposed Jurassic Fucus is shown in fig. 49, and briefly described in the Chapter dealing with fossil Bryophytes.

Several species of Flysch Algae have recently been referred by Rothpletz[367] to the Phaeophyceae under the provisional generic name Phycopsis, but they are of no special botanical interest.

The extremely interesting genus Nematophycus has lately been assigned by a Canadian author[368] to a position in the Phaeophyceae. Although the particular points on which he chiefly relies are not perhaps thoroughly established, there are certain considerations which lead us to include Nematophycus as a doubtful member of the present group of algae.

Nematophycus.

The stem attains a diameter of between 2 and 3 feet in the largest specimens; it is made up either of comparatively wide and loosely arranged tubes pursuing a slightly irregular vertical course accompanied by a plexus of much narrower tubes, or of tubes varying in diameter but not divisible into two distinct types. Rings of growth occur in some forms but not in others. Radially elongated or isodiametric spaces occur in the stem tissues in which the tubes are less abundant.

Reproductive organs unknown, with the possible exception of some very doubtful bodies described as spores.

In 1856 Sir William Dawson proposed the generic name Prototaxites for some large silicified trunks discovered in the Lower and Middle Devonian rocks of Canada. A few years later the same writer[369] published a detailed account of the new fossils and arrived at the conclusion that the Devonian stem showed definite points of affinity with the recent genus Taxus, and the generic name suggests that he regarded it as the type of Coniferous trees belonging to the sub-family Taxineae. The reasons for this determination were afterwards shown by Carruthers to be erroneous. Dawson thought he recognised pits and spiral thickenings in the walls of the tubular elements, as well as pointed ends in some of the latter. The spiral markings were in reality small hyphal tubes passing obliquely across the face of the wider tubes, and the apparent ends of the supposed tracheids were deceptive appearances due to the fact that the tubes had in some cases been cut through in an oblique direction. In 1870 Carruthers[370] expressed the opinion that Dawson’s Prototaxites was a “colossal fossil seaweed” and not a coniferous plant. The same author[371] in 1872 published a full and able account of the genus, and conclusively proved that Prototaxites could not be accepted as a Phanerogam; he brought forward almost convincing evidence in favour of including the genus among the algae. The name Prototaxites was now changed for that of Nematophycus. Carruthers compares the rings of growth in the fossil stems with those in the large Antarctic Lessonia stems, but he regards the histological characters as pointing to the Siphoneae as the most likely group of recent algae in which to include the Palaeozoic genus.

We may pass over various notes and additional contributions by Dawson, who did not admit the corrections to his original descriptions which Carruthers’ work supplied. In 1889 an important memoir appeared by Penhallow[372] of Montreal in which he confirmed Carruthers’ decision as to the algal nature of Prototaxites; he contributed some new facts to the previous account by Carruthers, and expressed himself in favour of regarding the fossil plant as a near ally of the recent Laminariae. The next addition to our botanical knowledge of this genus was made by Barber[373] who described a new specific type of Nematophycus—N. Storriei—found by Storrie in beds of Wenlock limestone age near Cardiff. Solms-Laubach[374], in a recent memoir on Devonian plants, recorded the occurrence of another species of this genus in Middle Devonian rocks near Gräfrath on the Lower Rhine. Lastly Penhallow[375], in describing a new species, lays stress on the resemblance of some of the tubular elements in the stem to the sieve-hyphae of the recent seaweeds Macrocystis and Laminaria. He concludes that the new facts he records make it clear that Nematophycus “is an alga, and of an alliance with the Laminarias.” The recent evidence brought forward by Penhallow is not entirely satisfactory; the drawings and descriptions of the supposed trumpet-shaped sieve-hyphae are not conclusive. On the whole it is probably the better course to speak of Nematophycus as a possible ally of the brown algae rather than as an extinct type of the Siphoneae, but until our knowledge is more complete it is practically impossible to decide the exact position of this Siluro-Devonian genus.

Solms-Laubach[376] has suggested that the generic name Nematophyton, used by Penhallow in preference to Carruthers’ term Nematophycus, is the more suitable as being a neutral designation and not one which assumes a definite botanical position. In view of the nature of the evidence in favour of the algal affinities of the fossil, the reasons for discarding Carruthers’ original name are hardly sufficient.

Before discussing more fully the distribution and botanical position of Nematophycus we may describe at length one of the best known species, and give a short account of some other forms.

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1. Nematophycus Logani (Daws.). Fig. 39, A–E. The stem possesses well marked concentric rings of growth due to a periodic difference in size of the large tubular elements. The tissues consist of two distinct kinds of tubular elements, the larger tubes loosely arranged and pursuing a fairly regular longitudinal course, and having a diameter of 13–35µ; the smaller tubes, with a diameter of 5–6µ, ramify in different directions and form a loose plexus among the larger and more regularly disposed elements. Branching occurs in both kinds of tubes; septa have been recognised only in the smaller tubes. Irregular and discontinuous radial spaces traverse the stem tissues, having a superficial resemblance in their manner of occurrence to the medullary rays of the higher plants.

The best specimens of this species were obtained by Sir William Dawson from the Devonian Sandstones of Gaspé in New Brunswick. The largest stems had a diameter of 3 feet and reached a length of several feet[377]; in some examples Dawson found lateral appendages attached to the stem which he described as “spreading roots.” Externally the specimens were occasionally covered with a layer of friable coal, and internally the tissues were found to be more or less perfectly preserved by the infiltration of a siliceous solution. Most of the examples of Nematophycus from Britain and Germany are much smaller and less perfectly preserved than those from Canada. The Peter Redpath Museum, Montreal, contains several very large blocks of Nematophycus, in many of which one sees the concentric rings of growth clearly etched out by weathering agents in a cross section of a large stem.

In fig. 39, A, a sketch is given of a thin transverse section of a stem, drawn natural size. The lines of growth are clearly seen, and as in coniferous stems the breadth of the concentric zones varies considerably. The short lines traversing the tissues in a radial direction represent the medullary-ray-like spaces referred to in the specific diagnosis. A transverse section examined under a low-power objective presents the appearance of a number of thick-walled and comparatively wide tubes loosely arranged; they may be in contact or separated from one another. If the microscope be carefully focussed through the thickness of the section the transversely-cut tubes appear to move laterally, producing a curiously dazzling effect if the objective is raised or lowered rapidly. This lateral movement is due to the undulating vertical course of the tubes. Under a higher power the lighter-coloured matrix in which the tubes are embedded shows a number of very much smaller and thinner-walled hyphal elements; some of these are cut across transversely, others more or less obliquely and others again longitudinally. These smaller tubes constitute an irregular plexus surrounding and ramifying between the larger elements. The diameter of the larger tubes decreases for a certain distance in a radial direction as seen in a transverse section, and this change in size gives rise to the appearance of concentric lines indicating periodic changes in growth.

Fig. 39. Nematophycus Logani (Daws.). A. Part of a transverse section from a specimen in the British Museum. (Nat. size.) B. Transverse section from specimens in Mr Barber’s possession. C. Longitudinal section. (B and C × 160.) D. Transverse section showing a radial space. E. Transverse section; a few ‘cells’ more highly magnified. D and E from a specimen in the British Museum.

The radial spaces are characterised by the partial absence of the larger tubes, and as seen in longitudinal sections these spaces constitute regions in which the smaller tubes branch very freely. Fig. 39, B, represents a small piece of a transverse section seen under a fairly high power. In fig. 39, C, the tubes are seen in longitudinal section. The larger elements are unseptate and not very regular in their vertical course through the stem; the smaller elements are seen as fine tubes lying between and across the larger tubes. In the sections I have examined no undoubted transverse septa were detected in any of the tubular elements.

The question as to the possible connection between the larger and smaller elements is one which is not as yet satisfactorily disposed of. Penhallow[378] regards the finer hyphal elements as branches of the larger tubes, but Barber[379], who has carefully examined good material of Nematophycus Logani, was unable to detect any organic connection between the two. My own observations are in accord with those of Barber. Further details and numerous figures of this species of Nematophycus will be found in the memoirs of Carruthers, Penhallow and Barber.

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Some specimens of silicified Nematophycus stems afford particularly instructive examples of the state of preservation or method of mineralisation as a source of error in histological work. The sketches reproduced in fig. 39, D and E, were made from a section of a large specimen of Nematophycus in the British Museum. In fig. D we have one of the radial spaces containing some indistinct small elements, the tissue surrounding the space appears to consist of polygonal cells suggesting ordinary parenchymatous tissue. In fig. E a few of these ‘cells’ are seen more clearly, they have black and ragged walls, and often contain very small and faint circles of which the precise nature is uncertain. The true interpretation of this form of structure was first supplied by Penhallow[380]. The black network simulating parenchymatous tissue consists of the substance of Nematophycus tubes which has been completely redistributed during fossilisation and collected along fairly regular lines, as seen in figs. D and E. The original structure has been almost completely destroyed, and the material composing the walls of the large tubes has finally been rearranged as a network, interrupted here and there by the characteristic radial spaces which remain as evidence of the original Nematophycus characters. It is possible in some cases to trace every gradation from sections exhibiting the normal structure through those having the appearance shown in figs. D and E to others in which the structure is completely lost. Penhallow describes this method of fossilisation in N. crassus (Daws.); an examination of several specimens in the National Collection leads me to entirely confirm his general conclusions, and also to the opinion that N. Logani shows exactly the same manner of mineralisation as N. crassus. The chief point of interest as regards this method of preservation lies in the fact that a fossil described by Dawson[381] as Celluloxylon primaevum, and referred to as a probable conifer, is undoubtedly a badly preserved Nematophycus. Penhallow examined Dawson’s specimens and obtained convincing evidence of their identity with certain forms of highly altered Nematophycus stems.

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2. Nematophycus Storriei Barber. Fig. 40. The specimens on which Barber[382] founded this species were obtained by Mr Storrie from the Tymawr quarry near Cardiff, in beds of Wenlock age. The fragmentary nature of the material is largely compensated for by the excellence of the preservation. We may briefly define the species as follows:

The stem consists of separate interlacing undivided and usually unbranched tubes of varying diameter. Spaces more or less isodiametric in dimensions are scattered through the tissue. The spaces constitute regions in which the tubular elements branch freely.

The main distinguishing features of this British species are (i) the absence of two distinct and well-defined forms of tubular elements. The main part of the stem consists of thick walled tubes similar to those of N. Logani, but the spaces between them are occupied by thinner-walled and smaller tubes varying considerably in diameter; (ii) the form of the spaces which are not radially elongated as in N. Logani.

Fig. 40. Nematophycus Storriei Barb. Longitudinal section, from a photograph by Mr C. A. Barber. × 45.

Fig. 40 shows the undulating course of the tubes as seen in a longitudinal section; the black colour of some of the elements is due to the fact that the surface of the wall is seen, while in the lighter-coloured portions of the tubes the wall has been cut through. The lighter patch about the middle of the figure shows the form of one of the spaces in which the tubes are freely branched.

In addition to the two species already described six others have been recorded, but with these we need not concern ourselves in detail. One of these species, N. Hicksi, was found by Dr Hicks[383] in the Denbighshire grits quarry of Pen-y-Glog near Corwen in North Wales. The position of these beds has recently been determined by Mr Lake[384] as corresponding to that of the Wenlock limestone. This species and N. Storriei are both Silurian examples of the genus. It is possible, as Barber has suggested, that the specimens described under these two names should be referred to one species. The specimens found by Hicks were small and imperfectly preserved fragments; Etheridge has given a full description of their structure, and Barber has subsequently examined the material. The preservation is not such as will admit of any very precise specific diagnosis; the fragments are correctly referred to Nematophycus, but their specific characters cannot be clearly determined.

Solms-Laubach[385] has described some fragments of another species of Nematophycus from the Devonian rocks of the Lower Rhine. His specimens are chiefly interesting as extending the geographical range of the genus, and as affording examples of a curious method of preservation. The specimens obtained were small fragments, flattened and very dark brown in colour. The tubular elements consisted of an external membrane of black coal, enclosing a central core of dark red iron-oxide. On burning the fragment on a piece of platinum foil the coal composing the wall of the tubes was removed and the deep-red casts of the tube-cavities remained[386]. The investigation of the structural characters of this imperfect material was conducted by reflected light. Under certain conditions, when it is impossible to obtain thin sections for examination by transmitted light, it is possible to accomplish much, as shown by Solms-Laubach’s work, by means of observation with direct light.

The last species to be noticed is Nematophycus Ortoni recently described by Penhallow. There are no concentric rings of growth, no radial spaces and no smaller hyphae in the tissues of this type of stem. In longitudinal section, the tubes show occasional local expansions of the lumen which Penhallow compares with the ‘trumpet-hyphae’ of some recent brown algae. No actual sieve-plates or transverse walls have been detected, but the general appearance of the tubes is considered to afford distinct evidence of the original existence of such walls. The figures accompanying the description do not carry conviction as to the correctness of the reference of the tubes to imperfectly preserved sieve-hyphae.

The following list, taken, with a few alterations, from Penhallow’s memoir[387], shows the geographical and geological range of the species of Nematophycus hitherto recorded.

Nematophycus Logani (Daws.)		Lower Devonian of Gaspé.
		Silurian [Wenlock] of England.
		Silurian of New Brunswick.
N. Hicksi (Eth.)		Silurian. (Wenlock) of N. Wales.
N. crassus (Daws.)[388]		Middle Devonian of Gaspé and New York.
N. laxus (Daws.)		Lower Devonian of Gaspé.
N. tenuis (Daws.)		Lower Devonian of Gaspé.
N. Storriei (Barb.)		Silurian (Wenlock) of Wales (Cardiff).
N. dechenianus (Pied.)		Upper Devonian of Germany (Gräfrath).
N. Ortoni (Pen.)		Upper Erian of Ohio.

In summing up our information as to the structure of Nematophycus we find there are certain points not definitely settled, and which are of considerable importance. The few recorded instances of spore-like bodies by Penhallow and Barber are not satisfactory; we are still ignorant of the nature of the reproductive organs. Such instances of lateral appendages as have been referred to do not throw much light on the habit of the plant. So far as we know at present the stem of Nematophycus was not differentiated internally into a cortical and central region. It may be that the specimens have been only partially preserved, and the coaly layer which occasionally surrounds a stem may represent a carbonised cortex which has never been petrified. The large and loosely arranged tubes constitute the chief characteristic feature of the genus; in some cases (N. Logani) there is an accompanying plexus of smaller hyphae, in others (N. Storriei) there is no definite division of the tissue into two sets of tubes of uniform size, and in N. Ortoni the tubular elements are all of the large type.

Penhallow has recognised the branching of large tubes in N. Logani and N. crassus giving rise to the small hyphal elements. In most specimens, however, no such mode of origin of the smaller tubes can be detected. The spaces which interrupt the homogeneity of the tissues in some forms have been described as branching depots, on account of the frequent occurrence in these areas of much branched hyphae. The function of these spaces (fig. 39, D, and fig. 40) may be connected with aeration of the stem-tissues.

As Carruthers first pointed out the unseptate nature of the elements and the occurrence of large and small tubes forming a comparatively lax tissue suggested affinities with such recent genera as Penicillus, Halimeda, Udotea and other members of the Siphoneae. In those fossil stems which possess tubes of two distinct sizes, we cannot as a rule trace any organic connection between the two sets of tubular elements. Transverse septa have been detected in the tubes of some specimens of N. Logani. These considerations and the large size and habit of growth of the stem leave one sceptical as to the wisdom of assigning the fossil genus to the Siphoneae. On the other hand, apart from the doubtful sieve-hyphae of Penhallow, the manner of growth of the plant, the concentric rings, marked by a decrease in the diameter of the tubes, the lax arrangement and irregular course of the elements, afford points of agreement with some recent Phaeophyceae. The stem of a Laminaria (fig. 29) or of a Lessonia are the most obvious structures with which to compare Nematophycus. The medullary region of a Laminaria or Fucus and of other genera presents a certain resemblance to the tissues of the fossil stems. On the whole we may be content to leave Nematophycus for the present as probably an extinct type of alga, more closely allied to the large members of the Phaeophyceae than to any other recent seaweeds.

Pachytheca.

(A fossil of uncertain affinity.)

There is another fossil occasionally associated with Nematophycus and referred by many writers to the Algae, which calls for a brief notice. Pachytheca is too doubtful a genus to justify a detailed treatment in the present work. Although, as I have elsewhere suggested[389], we are hardly in a position to speak with any degree of certainty as to its affinity, it is not improbable that it may eventually be shown to be an alga.

Without attempting a full diagnosis of the genus, we may briefly refer to its most striking characters.

Pachytheca usually occurs in the form of small spherical bodies, about ·5 cm. in diameter, in Old Red Sandstone or Silurian rocks. In section a single sphere is found to consist of two well marked regions; in the centre, of a number of ramifying and irregularly placed narrow tubes, and in the peripheral or cortical region, of numerous regular and radially disposed simple or forked septate tubes. The tubular elements of the two regions are in organic connection.

The name was proposed by Sir Joseph Hooker for some specimens found by Dr Strickland[390] in the Ludlow bone-bed (Silurian) of Woolhope and May-Hill. Examples were subsequently recorded from the Wenlock limestone of Malvern and from Silurian and Old Red Sandstone rocks of other districts. Hicks[391] found Pachytheca in the Pen-y-Glog grits of Corwen in association with Nematophycus, and the two fossils have been found together elsewhere. This association led to the suggestion that Pachytheca might be the sporangium of Nematophycus, and Dawson[392], in conformity with his belief in the coniferous character of the latter plant, referred to Pachytheca as a true seed.

The best sections of this fossil have been prepared with remarkable skill by Mr Storrie of Cardiff; they were carefully examined and described by Barber in two memoirs[393] published in the Annals of Botany, the account being illustrated by several well executed drawings and microphotographs.

Among other difficulties to contend against in the interpretation of Pachytheca there is that of mineralisation. The preservation is such as to render the discrimination of original structure as distinct from structural features of secondary origin, consequent on a particular manner of crystallisation of the siliceous material, a matter of considerable difficulty.

Suggestions as to the nature of Pachytheca have been particularly numerous; it has been referred to most classes of plants and relegated by some writers to the animal kingdom. The most recent addition to our knowledge of this problematic fossil was the discovery of a specimen by Mr Storrie in which the Pachytheca sphere rested in a small cup, like an acorn fruit in its cupule. This specimen was figured and described by Mr George Murray[394] in 1895; he expresses the opinion that the discovery makes the taxonomic position of the genus still more obscure. Solms-Laubach briefly refers to Pachytheca in connection with Nematophycus, and regards its precise nature almost as much an unsolved riddle now as it was when first discovered. For a fuller account of this fossil reference must be made to the contributions of Hooker[395], Barber[396] and others. The literature is quoted by Barber and more recently by Solms-Laubach[397]. There are several specimens and microscopic sections of Pachytheca in the geological and botanical departments of the British Museum. The genus has been recorded from Shropshire, North Wales, Malvern, Herefordshire, Perthshire and other British localities, as well as from Canada; it occurs in both Silurian and Old Red Sandstone rocks.

Algites.

A generic name for those fossils which in all probability belong to the class Algae, but which, by reason of the absence of reproductive organs, internal structure, or characters of a trustworthy nature in the determination of affinity, cannot be referred with any degree of certainty to a particular recent genus or family.

This term was suggested in 1894[398] as a provisional and comprehensive designation under which might be included such impressions or casts as might reasonably be referred to Algae. The practice of applying to alga-like fossils names suggestive of a definite alliance with recent genera is as a rule unsound. It would simplify nomenclature, and avoid the multiplication of generic names, if the term Algites were applied to such algal fossils from rocks of various ages as afford no trustworthy data by which their family or generic affinity can be established.