CHAPTER IX.
THE LOWER SILURIAN PERIOD.
The great system of deposits to which Sir Roderick Murchison applied the name of "Silurian Rocks" reposes directly upon the highest Cambrian beds, apparently without any marked unconformity, though with a considerable change in the nature of the fossils. The name "Silurian" was originally proposed by the eminent geologist just alluded to for a great series of strata lying below the Old Red Sandstone, and occupying districts in Wales and its borders which were at one time inhabited by the "Silures," a tribe of ancient Britons. Deposits of a corresponding age are now known to be largely developed in other parts of England, in Scotland, and in Ireland, in North America, in Australia, in India, in Bohemia, Saxony, Bavaria, Russia, Sweden and Norway, Spain, and in various other regions of less note. In some regions, as in the neighbourhood of St Petersburg, the Silurian strata are found not only to have preserved their original horizontality, but also to have retained almost unaltered their primitive soft and incoherent nature. In other regions, as in Scandinavia and many parts of North America, similar strata, now consolidated into shales, sandstones, and limestones, may be found resting with a very slight inclination on still older sediments. In a great many regions, however, the Silurian deposits are found to have undergone more or less folding, crumpling, and dislocation, accompanied by induration and "cleavage" of the finer and softer sediments; whilst in some regions, as in the Highlands of Scotland, actual "metamorphism" has taken place. In consequence of the above, Silurian districts usually present the bold, rugged, and picturesque outlines which are characteristic of the older "Primitive" rocks of the earth's crust in general. In many instances, we find Silurian strata rising into mountain-chains of great grandeur and sublimity, exhibiting the utmost diversity of which rock-scenery is capable, and delighting the artist with endless changes of valley, lake, and cliff. Such districts are little suitable for agriculture, though this is often compensated for by the valuable mineral products contained in the rocks. On the other hand, when the rocks are tolerably soft and uniform in their nature, or when few disturbances of the crust of the earth have taken place, we may find Silurian areas to be covered with an abundant pasturage or to be heavily timbered.
Under the head of "Silurian Rocks," Sir Roderick Murchison included all the strata between the summit of the "Longmynd." beds and the Old Red Sandstone, and he divided these into the two great groups of the Lower Silurian and Upper Silurian. It is, however, now generally admitted that a considerable portion of the basement beds of Murchison's Silurian series must be transferred—if only upon palæontological grounds—to the Upper Cambrian, as has here been done; and much controversy has been carried on as to the proper nomenclature of the Upper Silurian and of the remaining portion of Murchison's Lower Silurian. Thus, some would confine the name "Silurian" exclusively to the Upper Silurian, and would apply the name of "Cambro-Silurian" to the Lower Silurian, or would include all beds of the latter age in the "Cambrian" series of Sedgwick. It is not necessary to enter into the merits of these conflicting views. For our present purpose, it is sufficient to recognise that there exist two great groups of rocks between the highest Cambrian beds, as here defined, and the base of the Devonian or Old Red Sandstone. These two great groups are so closely allied to one another, both physically and palæontologically, that many authorities have established a third or intermediate group (the "Middle Silurian"), by which a passage is made from one into the other. This method of procedure involves disadvantages which appear to outweigh its advantages; and the two groups in question are not only generally capable of very distinct stratigraphical separation, but at the same time exhibit, together with the alliances above spoken of, so many and such important palæontological differences, that it is best to consider them separately. We shall therefore follow this course in the present instance; and pending the final solution of the controversy as to Cambrian and Silurian nomenclature, we shall distinguish these two groups of strata as the "Lower Silurian" and the "Upper Silurian."
The Lower Silurian Rocks are known already to be developed in various regions; and though their general succession in these areas is approximately the same, each area exhibits peculiarities of its own, whilst the subdivisions of each are known by special names. All, therefore, that can be attempted here, is to select two typical areas—such as Wales and North America and to briefly consider the grouping and divisions of the Lower Silurian in each.
In Wales, the line between the Cambrian and Lower Silurian is somewhat ill-defined, and is certainly not marked by any strong unconformity. There are, however; grounds for accepting the line proposed, for palæontological reasons, by Dr Hicks, in accordance with which the Tremadoc Slates ("Lower Tremadoc" of Salter) become the highest of the Cambrian deposits of Britain. If we take this view, the Lower Silurian rocks of Wales and adjoining districts are found to have the following general succession from below upwards (fig. 34):—
1. The Arenig Group.—This group derives its name from the Arenig mountains, where it is extensively developed. It consists of about 4000 feet of slates, shales, and flags, and is divisible into a lower, middle, and upper division, of which the former is often regarded as Cambrian under the name of "Upper Tremadoc Slates."
2. The Llandeilo Group.—The thickness of this group varies from about 4000 to as much as 10,000 feet; but in this latter case a great amount of the thickness is made up of volcanic ashes and interbedded traps. The sedimentary beds of this group are principally slates and flags, the latter occasionally with calcareous bands; and the whole series can be divided into a lower, middle, and upper Llandeilo division, of which the last is the most important. The name of "Llandeilo" is derived from the town of the same name in Wales, where strata of this age were described by Murchison.
3. The Caradoc or Bala Group.—The alternative names of this group are also of local origin, and are derived, the one from Caer Caradoc in Shropshire, the other from Bala in Wales, strata of this age occurring in both localities. The series is divided into a lower and upper group, the latter chiefly composed of shales and flags, and the former of sandstones and shales, together with the important and interesting calcareous band known as the "Bala Limestone." The thickness of the entire series varies from 4000 to as much as 12,000 feet, according as it contains more or less of interstratified igneous rocks.
4. The Llandovery Group (Lower Llandovery of Murchison).—This series, as developed near the town of Llandovery, in Caermarthenshire, consists of less than 1000 feet of conglomerates, sandstones, and shales. It is probable, however, that the little calcareous band known as the "Hirnant Limestone," together with certain pale-coloured slates which lie above the Bala Limestone, though usually referred to the Caradoc series, should in reality be regarded as belonging to the Llandovery group.
The general succession of the Lower Silurian strata of Wales and its borders, attaining a maximum thickness (along with contemporaneous igneous matter) of nearly 30,000 feet, is diagramatically represented in the annexed sketch-section (fig. 34):—
GENERALIZED SECTION OF THE LOWER SILURIAN ROCKS OF WALES.
Fig. 34.
In North America, both in the United States and in Canada, the Silurian rocks are very largely developed, and may be regarded as constituting an exceedingly full and typical series of the deposits of this period. The chief groups of the Silurian rocks of North America are as follows, beginning, as before, with the lowest strata, and proceeding upwards (fig. 35):—
1. Quebec Group.—This group is typically developed in the vicinity of Quebec, where it consists of about 5000 feet of strata, chiefly variously-coloured shales, together with some sandstones and a few calcareous bands. It contains a number of peculiar Graptolites, by which it can be identified without question with the Arenig group of Wales and the corresponding Skiddaw Slates of the North of England. It is also to be noted that numerous Trilobites of a distinct Cambrian facies have been obtained in the limestones of the Quebec group, near Quebec. These fossils, however, have been exclusively obtained from the limestones of the group; and as these limestones are principally calcareous breccias or conglomerates, there is room for believing that these primordial fossils are really derived, in part at any rate, from fragments of an upper Cambrian limestone. In the State of New York, the Graptolitic shales of Quebec are wanting; and the base of the Silurian is constituted by the so-called "Calciferous Sand-rock" and "Chazy Limestone."[11] The first of these is essentially and typically calcareous, and the second is a genuine limestone.
[Footnote 11: The precise relations of the Quebec shales with Graptolites (Levis Formation) to the Calciferous and Chazy beds are still obscure, though there seems little doubt but that the Quebec Shales are superior to the Calciferous Sand-rock.]
2. The Trenton Group.—This is an essentially calcareous group, the various limestones of which it is composed being known as the "Bird's-eye," "Black River," and "Trenton" Limestones, of which the last is the thickest and most important. The thickness of this group is variable, and the bands of limestone in it are often separated by beds of shale.
3. The Cincinnati Group (Hudson River Formation[12]).—This group consists essentially of a lower series of shales, often black in colour and highly charged with bituminous matter (the "Utica Slates "), and of an upper series of shales, sandstones, and limestones (the "Cincinnati" rocks proper). The exact parallelism of the Trenton and Cincinnati groups with the subdivisions of the Welsh Silurian series can hardly be stated positively. Probably no precise equivalency exists; but there can be no doubt but that the Trenton and Cincinnati groups correspond, as a whole, with the Llandeilo and Caradoc groups of Britain. The subjoined diagrammatic section (fig. 35) gives a general idea of the succession of the Lower Silurian rocks of North America:— GENERALIZED SECTION OF THE LOWER SILURIAN ROCKS OF NORTH AMERICA.
Fig. 35.
[Footnote 12: There is some difficulty about the precise nomenclature of this group. It was originally called the "Hudson River Formation;" but this name is inappropriate, as rocks of this age hardly touch anywhere the actual Hudson River itself, the rocks so called formerly being now known to be of more ancient date. There is also some want of propriety in the name of "Cincinnati Group," since the rocks which are known under this name in the vicinity of Cincinnati itself are the representatives of the Trenton Limestone, Utica Slates, and the old Hudson River group, inseparably united in what used to be called the "Blue Limestone Series.">[.
Of the life of the Lower Silurian period we have record in a vast number of fossils, showing that the seas of this period were abundantly furnished with living denizens. We have, however, in the meanwhile, no knowledge of the land-surfaces of the period. We have therefore no means of speculating as to the nature of the terrestrial animals of this ancient age, nor is anything known with certainty of any land-plants which may have existed. The only relics of vegetation upon which a positive opinion can be expressed belong to the obscure group of the "Fucoids," and are supposed to be the remains of sea-weeds. Some of the fossils usually placed under this head are probably not of a vegetable
Fig. 36.—Licrophycus Ottawaensis a "Fucoid," from the Trenton Limestone (Lower Silurian) of Canada. (After Billings.) nature at all, but others (fig. 36) appear to be unquestionable plants. The true affinities of these, however, are extremely dubious. All that can be said is, that remains which appear to be certainly vegetable, and which are most probably due to marine plants, have been recognised nearly at the base of the Lower Silurian (Arenig), and that they are found throughout the series whenever suitable conditions recur.
The Protozoans appear to have flourished extensively in the Lower Silurian seas, though to a large extent under forms which are still little understood. We have here for the first time the appearance of Foraminifera of the ordinary type—one of the most interesting observations in this collection being that made by Ehrenberg, who showed that the Lower Silurian sandstones of the neighbourhood of St Petersburg contained casts in glauconite of Foraminiferous shells, some of which are referable to the existing genera Rotalia and Texularia. True Sponges, belonging to that section of the group in which the skeleton is calcareous, are also not unknown, one of the
Fig. 37.—Astylospongia prœmorsa, cut vertically so as to exhibit the canal-system in the interior. Lower Silurian, Tennessee. (After Ferdinand Rœmer.) most characteristic genera being Astylospongia (fig. 37). In this genus are included more or less globular, often lobed sponges, which are believed not to have been attached to foreign bodies. In the form here figured there is a funnel-shaped cavity at the summit; and the entire mass of the sponge is perforated, as in living examples, by a system of canals which convey the sea-water to all parts of the organism. The canals by which the sea-water gains entrance open on the exterior of the sphere, and those by which it again escapes from the sponge open into the cup-shaped depression at the summit.
The most abundant, and at the same time the least understood, of Lower Silurian Protozoans belong, however, to the genera Stromatopora and Receptaculites, the structure of which can merely be alluded to here. The specimens of Stromatopora (fig. 38) occur as hemispherical, pear-shaped, globular, or irregular masses, often of very considerable size, and sometimes demonstrably attached to foreign bodies. In their structure these masses consist of numerous thin calcareous laminæ, usually arranged concentrically, and separated by narrow interspaces. These interspaces are generally crossed by numerous vertical calcareous pillars, giving the vertical section of the fossil a lattice-like appearance. There are also usually minute pores in the concentric laminæ, by which the successive interspaces are
Fig. 38.—A small and perfect specimen of Stromatopora rugosa, of the natural size, from the Trenton Limestone of Canada. (After Billings.) placed in communication; and sometimes the surface presents large rounded openings, which appear to correspond with the water-canals of the Sponges. Upon the whole, though presenting some curious affinities to the calcareous Sponges, Stromatopora is perhaps more properly regarded as a gigantic Foraminifer. If this view be correct, it is of special interest as being probably the nearest ally of Eozoön, the general appearance of the two being strikingly similar, though their minute structure is not at all the same. Lastly, in the fossils known as Receptaculites and Ischadites we are also presented with certain singular Lower Silurian Protozoans, which may with great probability be regarded as gigantic Foraminifera. Their structure is very complex; but fragments are easily recognised by the fact that the exterior is covered with numerous rhomboidal calcareous plates, closely fitting together, and arranged in peculiar intersecting curves, presenting very much the appearance of the engine-turned case of a watch.
Passing next to the sub-kingdom of Cœlenterate animals (Zoophytes, Corals, &c.), we find that this great group, almost or wholly absent in the Cambrian, is represented in Lower Silurian deposits by a great number of forms belonging on the one hand to the true Corals, and en the other hand to the singular family of the Graptolites. If we except certain plant-like fossils which probably belong rather to the Sertularians or the Polyzoans (e.g., Dictyonema, Dendrograptus, &c.), the family of the Graptolites may be regarded as exclusively Silurian in its distribution. Not only is this the case, but it attained its maximum development almost upon its first appearance, in the Arenig Rocks; and whilst represented by a great variety of types in the Lower Silurian; it only exists in the Upper Silurian in a much diminished form. The Graptolites (Gr. grapho, I write; lithos, stone) were so named by Linnæus, from the resemblance of some of them to written or pencilled marks upon the stone, though the great naturalist himself did not believe them to be true fossils at all. They occur as linear or leaf-like bodies, sometimes simple, sometimes compound and branched; and no doubt whatever can be entertained as to their being the skeletons of composite organisms, or colonies of semi-independent animals united together by a common fleshy trunk, similar to what is observed in the colonies of the existing Sea-firs (Sertularians). This fleshy trunk or common stem of the colony was protected by a delicate horny sheath, and it gave origin to the little flower-like "polypites," which constituted the active element of the whole assemblage. These semi-independent beings were, in turn, protected each by a little horny cup or cell, directly connected with the common sheath below, and terminating above in an opening through which the polypite could protrude its tentacled head or could again withdraw itself for safety. The entire skeleton, again, was usually, if not universally, supported by a delicate horny rod or "axis," which appears to have been hollow, and which often protrudes to a greater or less extent beyond one or both of the extremities of the actual colony.
The above gives the elementary constitution of any Graptolite, but there are considerable differences as to the manner in which these elements are arranged and combined. In some forms the common stem of the colony gives origin to but a single row of cells on one side. If the common stem is a simple, straight, or slightly-curved linear body, then we have the simplest form of Graptolite known (the genus Monograptus); and it is worthy of note that these simple types do not come into existence till comparatively late (Llandeilo), and last nearly to the very close of the Upper Silurian. In other cases, whilst there is still but a single row of cells, the colony may consist of two of these simple stems springing from a common point, as in the so-called "twin Graptolites" (Didymograptus, fig. 40). This type is
Fig. 39.—Dichograptus octobrachiatus, a branched, "unicellular" Graptolite from the Skiddaw and Quebec Groups (Arenig). (After Hall.) entirely confined to the earlier portion of the Lower Silurian period (Arenig and Llandeilo). In other cases, again, there may be four of such stems springing from a central point (Tetragraptus). Lastly, there are numerous complex forms (such as Dichograptus, Loganograptus, &c.) in which there are eight or more of these simple branches, all arising from a common centre (fig. 39), which is sometimes furnished with a singular horny disc. These complicated branching forms, as well as the Tetragrapti, are characteristic of the horizon of the Arenig group. Similar forms, often specifically identical, are found at this horizon in Wales, in the great series of the Skiddaw Slates of the north of England, in the Quebec group in Canada, in equivalent beds in Sweden, and in certain gold-bearing slates of the same age in Victoria in Australia.
In another great group of Graptolites (including the genera Diplograptus, Dicranograptus, Climacograptus, &c.) the common stem of the colony gives origin, over part or the whole or its length, to two rows of cells, one on each side (fig. 41). These "double-celled" Graptolites are highly characteristic of the Lower Silurian deposits; and, with an exception more apparent than real in Bohemia, they are exclusively confined to strata of Lower Silurian age, and are not known to occur in the Upper Silurian.
Fig. 40.—Central portion of the colony of Didymegraptus divaricatus, Upper Llandeilo, Dumfresshire. (Original.) Lastly, there is a group of Graptolites (Phyllograptus, fig. 42) in which the colony is leaf-like in form, and is composed
Fig. 41.—Examples of Diplograptus pristis, showing variations in the appendages at the base. Upper Llandeilo, Dumfriesshire. (Original.)
Fig. 42.—Group of individuals of Phyllograptus typus, from the Quebec group of Canada. (After Hall.) One of the four rows of cells is hidden on the under surface. of four rows of cells springing in a cross-like manner from the common stem. These forms are highly characteristic of the Arenig group.
The Graptolites are usually found in dark-coloured, often black shales, which sometimes contain so much carbon as to become "anthracitic." They may be simply carbonaceous; but they are more commonly converted into iron-pyrites, when they glitter with the brilliant lustre of silver as they lie scattered on the surface of the rock, fully deserving in their metallic tracery the name of "written stones." They constitute one of the most important groups of Silurian fossils, and are of the greatest value in determining the precise stratigraphical position of the beds in which they occur. They present, however, special difficulties in their study; and it is still a moot point as to their precise position in the zoological scale. The balance of evidence is in favour of regarding them as an ancient and peculiar group of the Sea-firs (Hydroid Zoophytes), but some regard them as belonging rather to the Sea-mosses (Polyzoa). Under any circumstances, they cannot be directly compared either with the ordinary Sea-firs or the ordinary Sea-mosses; for these two groups consist of fixed organisms, whereas the Graptolites were certainly free-floating creatures, living at large in the open sea. The only Hydroid Zoophytes or Polyzoans which have a similar free mode of existence, have either no skeleton at all, or have hard structures quite unlike the horny sheaths of the Graptolites.
The second great group of Cœlenterate animals (Actinozoa) is represented in the Lower Silurian rocks by numerous Corals. These, for obvious reasons, are much more abundant in regions where the Lower Silurian series is largely calcareous (as in North America) than in districts like Wales, where limestones are very feebly developed. The Lower Silurian Corals, though the first of their class, and presenting certain peculiarities, may be regarded as essentially similar in nature to existing Corals. These, as is well known, are the calcareous skeletons of animals—the so-called "Coral-Zoophytes"—closely allied to the common Sea-anemones in structure and habit. A simple coral (fig. 43) consists of a calcareous cup embedded in the soft tissues of the flower-like polype, and having at its summit a more or less deep depression (the "calice") in which the digestive organs are contained. The space within the coral is divided into compartments by numerous vertical calcareous plates (the "septa"), which spring from the inside of the wall of the cup, and of which some generally reach the centre. Compound corals, again (fig. 44), consist of a greater or less number of structures similar in structure to the above, but united together in different ways into a common mass. Simple corals, therefore, are the
Fig. 43.—Zaphrentis Stokesi, a simple "cup-coral," Upper Silurian, Canada. (After Billings.)
Fig. 44.—Upper surface of a mass of Strombodes pentagonus. Upper Silurian, Canada. (After Billings.) skeletons of single and independent polypes; whilst compound corals are the skeletons of assemblages or colonies of similar polypes, living united with one another another as an organic community.
In the general details of their structure, the Lower Silurian Corals do not differ from the ordinary Corals of the present day. The latter, however, have the vertical calcareous plates of the coral ("septa") arranged in multiples of six or five; whereas the former have these structures arranged in multiples of four, and often showing a cross-like disposition. For this reason, the common Lower Silurian Corals are separated to form a distinct group under the name of Rugose Corals or Rugosa. They are further distinguished by the fact that the cavity of the coral ("visceral chamber") is usually subdivided by more or less numerous horizontal calcareous plates or partitions, which divide the coral into so many tiers or storeys, and which are known as the "tabulæ" (fig. 45).
In addition to the Rugose Corals, the Lower Silurian rocks contain a number of curious compound corals, the tubes of which have either no septa at all or merely rudimentary ones, but which have the transverse partitions or "tabulæ" very highly developed. These are known as the Tabulate Corals; and recent researches on some of their existing allies (such as Heliopora) have shown that they are really allied to the modern Sea-pens, Organ-pipe Corals, and Red Coral, rather than to the typical stony Corals. Amongst the characteristic Rugose Corals of the Lower Silurian
Fig. 45.—Columnaria alveolata, a Rugose compound coral, with imperfect septa, but having the corallites partitioned off into storeys by "tabulæ." Lower Silurian, Canada. (After Billings.) may be mentioned species belonging to the genera Columnaria, Favistella, Streptelasma, and Zaphrentis; whilst amongst the "Tabulate" Corals, the principal forms belong to the genera Chœtetes, Halysites (the Chain-coral), Constellaria, and Heliolites. These groups of the Corals, however, attain a greater development at a later period, and they will be noticed more particularly hereafter.
[Footnote 13: The genus Caryocrinus is sometimes regarded as properly belonging to the Crinoids, but there seem to be good reasons for rather considering it as an abnormal form of Cystidean.]
Passing onto higher animals, we find that the class of the Echinodermata is represented by examples of the Star-fishes (Asteroidea), the Sea-lilies (Crinoidea), and the peculiar extinct group of the Cystideans (Cystoidea), with one or two of the Brittle-stars (Ophiuroidea)—the Sea-urchins (Echinoidea) being still wanting. The Crinoids, though in some places extremely numerous, have not the varied development that they possess in the Upper Silurian, in connection with which their structure will be more fully spoken of. In the meanwhile, it is sufficient to note that many of the calcareous deposits of the Lower Silurian are strictly entitled to the name of "Crinoidal limestones," being composed in great part of the detached joints, and plates, and broken stems, of these beautiful but fragile organisms (see fig. 12). Allied to the Crinoids are the singular creatures which are known as Cystideans (fig. 46). These are generally composed of a globular or ovate body (the "calyx"), supported upon a short stalk (the "column"), by which the organism was usually attached to some foreign body. The body was enclosed by closely-fitting calcareous plates, accurately jointed together; and the stem was made up of numerous distinct pieces or joints, flexibly united to each other by membrane. The
Fig. 46.—Group of Cystideans. A, Caryocrinus ornatus,[13] Upper Silurian, America; B, Pleurocystites squamosus, showing two short "arms," Lower Silurian, Canada; C, Pseudocrinus bifasciatus, Upper Silurian, England; D, Lepadocrinus Gebhartii, Upper Silurian, America. (After Hall, Billings, and Salter.) chief distinction which strikes one in comparing the Cystideans with the Crinoids is, that the latter are always furnished, as will be subsequently seen, with a beautiful crown of branched and feathery appendages, springing from the summit of the calyx, and which are composed of innumerable calcareous plates or joints, and are known as the "arms." In the Cystideans, on the other hand, there are either no "arms" at all, or merely short, unbranched, rudimentary arms. The Cystideans are principally, and indeed nearly exclusively, Silurian fossils; and though occurring in the Upper Silurian in no small numbers, they are pre-eminently characteristic of the Llandeilo-Caradoc period of Lower Silurian time. They commenced their existence, so far as known, in the Upper Cambrian; and though examples are not absolutely unknown in later periods, they are pre-eminently characteristic of the earlier portion of the Palæozoic epoch.
The Ringed Worms (Annelides) are abundantly represented in the Lower Silurian, but principally by tracks and burrows similar in essential respects to those which occur so commonly in the Cambrian formation, and calling for no special comment. Much more important are the Articulate animals, represented as heretofore, wholly by the remains of the aquatic group of the
Fig. 47.—Lower Silurian Crustaceans. a, Asaphus tyrannus, Upper Llandeilo; b. Ogygia Buchii, Upper Llandeilo; c, Trinucleus concentricus, Caradoc; d, Caryocaris Wrightii, Arenig (Skiddaw Slates); e, Beyrichia complicata, natural size and enlarged, Upper Llandeilo and Caradoc; f, Primitia strangulata, Caradoc: g. Head-shield of Calymene Blumenbachii, var. brevicapitata, Caradoc; h, Head-shield of Triarthrus Becki (Utica Slates), United States: i, Shield of Leperditia Canadensis, var. Josephiana, of the natural size, Trenton Limestone, Canada; j, The same, viewed from the front. (After Salter, M'Coy, Rupert Jones, and Dana.) Crustaceans. Amongst these are numerous little bivalved forms—such as species of Primitia (fig. 47, f), Beyrichia (fig. 47, e), and Leperditia (fig. 47, i and j). Most of these are very small, varying from the size of a pin's head up to that of a hemp seed; but they are sometimes as large as a small bean (fig. 47, i), and they are commonly found in myriads together in the rock. As before said, they belong to the same great group as the living Water-fleas (Ostracoda). Besides these, we find the pod-shaped head-shields of the shrimp-like Phyllopods—such as Caryocaris (fig. 47, d) and Ceratiocaris. More important, however, than any of these are the Trilobites, which may be considered as attaining their maximum development in the Lower Silurian. The huge Paradoxides of the Cambrian have now disappeared, and with them almost all the principal and characteristic "primordial" genera, save Olenus and Agnostus. In their place we have a great number of new forms—some of them, like the great Asaphus tyrannus of the Upper Llandeilo (fig. 47, a), attaining a length of a foot or more, and thus hardly yielding in the matter of size to their ancient rivals. Almost every subdivision of the Lower Silurian series has its own special and characteristic species of Trilobites; and the study of these is therefore of great importance to the geologist. A few widely-dispersed and characteristic species have been here figured (fig. 47); and the following may be considered as the principal Lower Silurian genera—Asaphus, Ogygia, Cheirurus, Ampyx, Caiymene, Trinucleus, Lichas, Illœnus, Æglina, Harpes, Remopleurides, Phacops, Acidaspis, and Homalonotus, a few of them passing upwards under new forms into the Upper Silurian.
Coming next to the Mollusca, we find the group of the Sea-mosses and Sea-mats (Polyzoa) represented now by quite a number of forms. Amongst these are examples of the true Lace-corals (Retepora and Fenestella), with their netted fan-like or funnel-shaped fronds; and along with these are numerous delicate encrusting forms, which grew parasitically attached to shells and corals (Hippothoa, Alecto, &c.); but perhaps the most characteristic forms belong to the genus Ptilodictya (figs. 48 and 49). In this group the frond is flattened, with thin striated edges, sometimes sword-like or scimitar-shaped, but often more or less branched; and it consists of two layers of cells, separated by a delicate membrane, and opening upon opposite sides. Each of these little chambers or "cells" was originally tenanted by a minute animal, and the whole thus constituted a compound organism or colony.
The Lamp-shells or Brachiopods are so numerous, and present such varied types, both in this and the succeeding period of the Upper Silurian, that the name of "Age of Brachiopods" has with justice been applied to the Silurian period as a whole. It would be impossible here to enter into details as to the many
Fig. 48.—Ptilodictya falciformis. a, Small specimen of the natural size; b, Cross-section, showing the shape of the frond; c, Portion of the surface, enlarged. Trenton Limestone and Cincinnati Group, America. (Original.)
Fig. 49.—A, Ptilodictya acuta; B. Ptilodictya Schafferi. a, Fragment, of the natural size; b, Portion, enlarged to show the cells. Cincinnati Group of Ohio and Canada. (Original.) different forms of Brachiopods which present themselves in the Lower Silurian deposits; but we may select the three genera Orthis, Strophomena, and Leptœna for illustration, as being specially characteristic of this period,
Fig. 50.—Lower Silurian Brachiopods. a and a', Orthis biforata, Llandeilo-Caradoc, Britain and America: b, Orthis flabellulum, Caradoc, Britain: c, Orthis subquadrata, Cincinnati Group, America; c', Interior of the dorsal valve of the same: d, Strophomena deltoidea, Llandeilo-Caradoc, Britain and America. (After Meek, Hall, and Salter.) though not exclusively confined to it. The numerous shells which belong to the extensive and cosmopolitan genus Orthis (fig. 50, a, b, c, and fig. 51, c and d), are usually more or less transversely-oblong or subquadrate, the two valves (as more or less in all the Brachiopods) of unequal sizes,
Fig. 51.—Lower Silurian Brachiopods, a, Strophomena alternata, Cincinnati Group, America; b, Strophomena filitexta, Trenton and Cincinnati Groups, America; c, Orthis testudinaria, Caradoc, Europe, and America; d, d', Orthis plicateila, Cincinnati Group, America; e, e', e'', Leptœna sericea, Llandeilo and Caradoc, Europe and America. (After Meek, Hall, and the Author.) generally more or less convex, and marked with radiating ribs or lines. The valves of the shell are united to one another by teeth and sockets, and there is a straight hinge-line. The beaks are also separated by a distinct space ("hinge-area"), formed in part by each valve, which is perforated by a triangular opening, through which, in the living condition, passed a muscular cord attaching the shell to some foreign object. The genus Strophomena (fig. 50, d, and 51, a and b) is very like Orthis in general character; but the shell is usually much flatter, one or other valve often being concave, the hinge-line is longer, and the aperture for the emission of the stalk of attachment is partially closed by a calcareous plate. In Leptœna, again (fig. 51, e), the shell is like Strophomena in many respects, but generally comparatively longer, often completely semicircular, and having one valve convex and the other valve concave. Amongst other genera of Brachiopods which are largely represented in the Lower Silurian rocks may be mentioned Lingula, Crania, Discina, Trematis, Siphonotreta, Acrotreta, Rhynchonella, and Athyris; but none of these can claim the importance to which the three previously-mentioned groups are entitled.
The remaining Lower Silurian groups of Mollusca can be but briefly glanced at here. The Bivalves (Lamellibranchiata) find numerous representatives, belonging to such genera as Modiolopsis, Ctenodonta, Orthonota, Palœarca, Lyrodesma,
Fig. 52.—Murchisonia gracilis, Trenton Limestone, America. (After Billings.) Ambonychia,and Cleidophorus. The Univalves (Gasteropoda) are also very numerous, the two most important genera being Murchisonia (fig. 52) and Pleurotomaria. In both these groups the outer lip of the shell is notched; but the shell in the former is elongated and turreted, whilst in the latter it is depressed. The curious oceanic Univalves known as the Heteropods are also very abundant, the principal forms belonging to Bellerophon and Maclurea. In the former (fig. 53) there is a symmetrical convoluted shell, like that of the Pearly Nautilus in shape, but without any internal partitions, and having the aperture often expanded and notched behind. The species of Maclurea (fig. 54) are found both in North America and in Scotland, and are exclusively confined to the Lower Silurian period, so far as known. They have the shell coiled into a flat spiral, the mouth being furnished with a very curious, thick, and solid lid or "operculum." The Lower Silurian Pteropods, or "Winged snails," are numerous, and belong principally to the genera Theca, Conularia, and Tentaculites, the last-mentioned of these often being extremely abundant in certain strata.
Lastly, the Lower Silurian Rocks have yielded a vast number of chambered shells, referable to animals which belong to the same great division as the Cuttle-fishes (the Cephalopoda), and of which the Pearly Nautilus is the only living representative at the present day. In this group of Cephalopods the animal
Fig. 53.—Different views of Bellerophon Argo, Trenton Limestone, Canada. (After Billings.) possesses a well-developed external shell, which is divided into chambers by shelly partitions ("septa"). The animal lives in the last-formed and largest chamber of the shell, to which it is organically connected by muscular attachments. The head is furnished with long muscular processes or "arms," and can be
Fig. 54.—Different views of Maclurea crenulata, Quebec Group, Newfoundland. (After Billings.) protruded from the mouth of the shell at will, or again withdrawn within it. We learn, also, from the Pearly Nautilus, that these animals must have possessed two pairs of breathing organs or "gills;" hence all these forms are grouped together under the name of the "Tetrabranchiate" Cephalopods (Gr. tetra, four; bragchia, gill). On the other hand, the ordinary Cuttle-fishes and Calamaries either possess an internal skeleton, or if they have an external shell, it is not chambered; their "arms" are furnished with powerful organs of adhesion in the form of suckers; and they possess only a single pair of gills. For this last reason they are termed the "Dibranchiate" Cephalopods (Gr. dis, twice; bragchia, gill). No trace of the true Cuttle-fishes has yet been found in Lower Silurian deposits; but the Tetrabranchiate group is represented by a great number of forms, sometimes of great size. The principal Lower Silurian genus is the well-known and widely-distributed Orthoceras (fig. 55). The shell in this genus agrees with that of the existing Pearly Nautilus, in consisting of numerous chambers separated by shelly partitions (or septa), the latter being perforated by a tube which runs the whole length of the shell after the last chamber, and is known as the "siphuncle" (fig. 56, s). The last chamber formed is the largest, and in it the animal lives. The chambers behind this are apparently filled with some gas secreted by the animal itself; and these are supposed to act as a kind of float, enabling the creature to move with ease under the weight of its shell. The various air-chambers, though the siphuncle passes through them, have no direct connection with one another; and it is believed that the animal has the power of slightly altering its specific gravity, and thus of rising or sinking in the water by driving additional fluid into the siphuncle or partially emptying it. The Orthoceras further agrees with the Pearly Nautilus in the fact that the partitions or septa separating the different air-chambers are
Fig. 55.—Fragment of Orthoceras crebriseptum, Cincinnati Group, North America, of the natural size. The lower figure section showing the air-chambers, and the form and position of the siphuncle. (After Billings.)
Fig. 56.—[14] Restoration of Orthoceras, the shell being supposed to be divided vertically, and only its upper part being shown. a, Arms; f, Muscular tube ("funnel") by which water is expelled from the mantle-chamber; c, Air-chambers; s, Siphuncle. simple and smooth, concave in front and convex behind, and devoid of the elaborate lobation which they exhibit in the Ammonites; whilst the siphuncle pierces the septa either in the centre or near it. In the Nautilus, however, the shell is coiled into a flat spiral; whereas in Orthoceras the shell is a straight, longer or shorter cone, tapering behind, and gradually expanding towards its mouth in front. The chief objections to the belief that the animal of the Orthoceras was essentially like that of the Pearly Nautilus are—the comparatively small size of the body-chamber, the often contracted aperture of the mouth, and the enormous size of some specimens of the shell. Thus, some Orthocerata have been discovered measuring ten or twelve feet in length, with a diameter of a foot at the larger extremity. These colossal dimensions certainly make it difficult to imagine that the comparatively small body-chamber could have held an animal large enough to move a load so ponderous as its own shell. To some, this difficulty has appeared so great that they prefer to believe that the Orthoceras did not live in its shell at all, but that its shell was an internal skeleton similar to what we shall find to exist in many of the true Cuttle-fishes. There is something to be said in favour of this view, but it would compel us to believe in the existence in Lower Silurian times of Cuttle-fishes fully equal in size to the giant "Kraken" of fable. It need only be added in this connection that the Lower Silurian rocks have yielded the remains of many other Tetrabranchiate Cephalopods besides Orthoceras. Some of these belong to Cyrtoceras, which only differs from Orthoceras in the bow-shaped form of the shell; others belong to Phragmoceras, Lituites, &c.; and, lastly; we have true Nautili, with their spiral shells, closely resembling the existing Pearly Nautilus.
[Footnote 14: This illustration is taken from a rough sketch made by the author many years ago, but he is unable to say from what original source it was copied.]
Whilst all the sub-kingdoms of the Invertebrate animals are represented in the Lower Silurian rocks, no traces of Vertebrate animals have ever been discovered in these ancient deposits, unless the so-called "Conodonts" found by Pander in vast numbers in strata of this age [15] in Russia should prove to be really of this nature. These problematical bodies are of microscopic size, and have the form of minute, conical, tooth-shaped spines, with sharp edges, and hollow at the base. Their original discoverer regarded them as the horny teeth of fishes allied to the Lampreys; but Owen came to the conclusion that they probably belonged to Invertebrates. The recent investigation of a vast number of similar but slightly larger bodies, of very various forms, in the Carboniferous rocks of Ohio, has led Professor Newberry to the conclusion that these singular fossils really are, as Pander thought, the teeth of Cyclostomatous fishes. The whole of this difficult question has thus been reopened, and we may yet have to record the first advent of Vertebrate animals in the Lower Silurian.
[Footnote 15: According to Pander, the "Conodonts" are found not only in the Lower Silurian beds, but also in the "Ungulite Grit" (Upper Cambrian), as well as in the Devonian and Carboniferous deposits of Russia. Should the Conodonts prove to be truly the remains of fishes, we should thus have to transfer the first appearance of vertebrates to, at any rate, as early a period as the Upper Cambrian.]
CHAPTER X.
THE UPPER SILURIAN PERIOD.
Having now treated of the Lower Silurian period at considerable length, it will not be necessary to discuss the succeeding group of the Upper Silurian in the same detail—the more so, as with a general change of species the Upper Silurian animals belong for the most part to the same great types as those which distinguish the Lower Silurian. As compared, also, as regards the total bulk of strata concerned, the thickness of the Upper Silurian is generally very much below that of the Lower Silurian, indicating that they represent a proportionately shorter period of time. In considering the general succession of the Upper Silurian beds, we shall, as before, select Wales and America as being two regions where these deposits are typically developed.
In Wales and its borders the general succession of the Upper Silurian rocks may be taken to be as follows, in ascending order (fig. 57):—
(1) The base of the Upper Silurian series is constituted by a series of arenaceous beds, to which the name of "May Hill Sandstone" was applied by Sedgwick. These are succeeded by a series of greenish-grey or pale-grey slates ("Tarannon Shales"), sometimes of great thickness; and these two groups of beds together form what may be termed the "May Hill Group" (Upper Llandovery of Murchison). Though not very extensively developed in Britain, this zone is one very well marked by its fossils; and it corresponds with the "Clinton Group" of North America, in which similar fossils occur. In South Wales this group is clearly unconformable to the highest member of the subjacent Lower Silurian (the Llandovery group); and there is reason to believe that a similar, though less conspicuous, physical break occurs very generally between the base of the Upper and the summit of the Lower Silurian.
(2) The Wenlock Group succeeds the May Hill group, and constitutes the middle member of the Upper Silurian. At its base it may have an irregular limestone ("Woolhope Limestone"), and its summit may be formed by a similar but thicker calcareous deposit ("Wenlock Limestone"); but the bulk of the group is made up of the argillaceous and shaly strata known as the "Wenlock Shale." In North Wales the Wenlock group is, represented by a great accumulation of flaggy and gritty strata (the "Denbighshire Flags and Grits"), and similar beds (the "Coniston Flags" and "Coniston Grits") take the same place in the north of England.
(3) The Ludlow Group is the highest member of the Upper Silurian, and consists typically of a lower arenaceous and shaly series (the "Lower Ludlow Rock") a middle calcareous member (the "Aymestry Limestone"), and an upper shaly and sandy series (the "Upper Ludlow Rock" and "Downton Sandstone"). At the summit, or close to the summit, of the Upper Ludlow, is a singular stratum only a few inches thick (varying from an inch to a foot), which contains numerous remains of crustaceans and fishes, and is well known under the name of the "bone-bed." Finally, the Upper Ludlow rock graduates invariably into a series of red sandy deposits, which, when of a flaggy character, are known locally as the "Tile-stones." These beds are probably to be regarded as the highest member of the Upper Silurian; but they are sometimes looked upon as passage-beds into the Old Red Sandstone, or as the base of this formation. It is, in fact, apparently impossible to draw any actual line of demarcation between the Upper Silurian and the overlying deposits of the Devonian or Old Red Sandstone series. Both in Britain and in America the Lower Devonian beds repose with perfect conformity upon the highest Silurian beds, and the two formations appear to pass into one another by a gradual and imperceptible transition.
The Upper Silurian strata of Britain vary from perhaps 3000 or 4000 feet in thickness up to 8000 or 10,000 feet. In North America the corresponding series, though also variable, is generally of much smaller thickness, and may be under 1000 feet. The general succession of the Upper Silurian deposits of North America is as follows:—
(1) Medina Sandstone.—This constitutes the base of the Upper Silurian, and consists of sandy strata, singularly devoid of life, and passing below in some localities into a conglomerate ("Oneida Conglomerate"), which is stated to contain pebbles derived from the older beds, and which would thus indicate an unconformity between the Upper and Lower Silurian.
(2) Clinton Group.—Above the Medina sandstone are beds of sandstone and shale, sometimes with calcareous bands, which constitute what is known as the "Clinton Group." The Medina and Clinton groups are undoubtedly the equivalent of the "May Hill Group" of Britain, as shown by the identity of their fossils.
GENERALIZED SECTION OF THE UPPER SILURIAN STRATA OF WALES AND SHROPSHIRE.
Fig. 57.
(3) Niagara Group.—This group consists typically of a series of argillaceous beds ("Niagara Shale") capped by limestones ("Niagara Limestone"); and the name of the group is derived from the fact that it is over limestones of this age that the Niagara river is precipitated to form the great Falls. In places the Niagara group is wholly calcareous, and it is continued upwards into a series of marls and sandstones, with beds of salt and masses of gypsum (the "Salina Group"), or into a series of magnesian limestones ("Guelph Limestones"). The Niagara group, as a whole, corresponds unequivocally with the Wenlock group of Britain.
(4) Lower Helderberg Group.—The Upper Silurian period in North America was terminated by the deposition of a series of calcareous beds, which derive the name of "Lower Helderberg" from the Helderberg mountains, south of Albany, and which are divided into several zones, capable of recognition by their fossils, and known by local names (Tentaculite Limestone, Water-lime, Lower Pentamerus Limestone, Delthyris Shaly Limestone, and Upper Pentamerus Limestone). As a whole, this series may be regarded as the equivalent of the Ludlow group of Britain, though it is difficult to establish any precise parallelism. The summit of the Lower Heiderberg group is constituted by a coarse-grained sandstone (the "Oriskany Sandstone"), replete with organic remains, which have to a large extent a Silurian facies. Opinions differ as to whether this sandstone is to be regarded as the highest bed of the Upper Silurian or the base of the Devonian. We thus see that in America, as in Britain, no other line than an artificial one can be drawn between the Upper Silurian and the overlying Devonian.
As regards the life of the Upper Silurian period, we have, as before, a number of so-called "Fucoids," the true vegetable nature of which is in many instances beyond doubt. In addition to these, however, we meet for the first time, in deposits of this age, with the remains of genuine land-plants, though our knowledge of these is still too scanty to enable us to construct any detailed picture of the terrestrial vegetation of the period. Some of these remains indicate the existence of the remarkable genus Lepidodendron—a genus which played a part of great importance in the forests of the Devonian and Carboniferous periods, and which may be regarded as a gigantic and extinct type of the Club-mosses (Lycopodiaceœ). Near the summit of the Ludlow formation in Britain there have also been found beds charged with numerous small globular bodies, which Dr Hooker has shown to be the seed-vessels or "sporangia" of Club-mosses. Principal Dawson further states that he has seen in the same formation fragments of wood with the structure of the singular Devonian Conifer known as Prototaxites. Lastly, the same distinguished observer has described from the Upper Silurian of North America the remains of the singular land-plants belonging to the genus Psilophyton, which will be referred to at greater length hereafter.
The marine life of the Upper Silurian is in the main constituted by types of animals similar to those characterising the Lower Silurian, though for the most part belonging to different species. The Protozoans are represented principally by Stromatopora and Ischadites, along with a number of undoubted sponges (such as Amphispongia, Astrœospongia, Astylospongia, and Palœomanon).
Amongst the Cœlenterates, we find the old group of Graptolites now verging on extinction. Individuals still remain numerous, but the variety of generic and specific types has now become greatly reduced. All the branching and complex forms of the Arenig, the twin-Graptolites
Fig. 58.—A, Monograptus priodon, slightly enlarged. B, Fragment of the same viewed from behind. C, Fragment of the same viewed in front, showing the mouths of the cellules. D, Cross-section of the same. From the Wenlock Group (Coniston Flags of the North of England). (Original.) and Dicranograpti of the Llandeilo, and the double-celled Diplograpti and Climacograpti of the Bala group, have now disappeared. In their place we have the singular Retiolites, with its curiously-reticulated skeleton; and several species of the single-celled genus Monograptus, of which a characteristic species (M. Priodon) is here figured. If we remove from this group the plant-like Dictyonemœ, which are still present, and which survive into the Devonian, no known species of Graptolite has hitherto been detected in strata higher in geological position than the Ludlow. This, therefore, presents us with the first instance we have as yet met with of the total disappearance and extinction of a great and important series of organic forms.
The Corals are very numerously represented in the Upper Silurian rocks some of the limestones (such as the Wenlock Limestone) being often largely composed of the skeletons of these animals. Almost all the known forms of this period belong to the two great divisions of the Rugose and Tabulate corals, the former being represented by species of Zaphrentis, Omphyma, Cystiphyllum, Strombodes, Acervularia, Cyathophyllum, &c.; whilst the latter belong principally to the genera Favosites, Chœtetes, Halysites, Syringopora, Heliolites, and Plasmopora. Amongst the Rugosa, the first appearance of the great and important genus Cyathophyllum, so characteristic of the Palæozoic period, is to be noted; and amongst the Tabulata we have similarly the first appearance, in force at any rate, of the widely-spread genus Favosites—the "Honeycomb-corals." The "Chain-corals" (Halysites), figured below (fig. 59), are also very common examples of the Tabulate corals during this period, though they occur likewise in the Lower Silurian.
Amongst the Echinodermata, all those orders which have hard parts capable of ready preservation are more or less largely
Fig. 59.—a, Halysites catenularia, small variety, of the natural size; b, Fragment of a large variety of the same, of the natural size; c, Fragment of limestone with the tubes of Halysites agglomerata, of the natural size; d, Vertical section of two tubes of the same, showing the tabulæ, enlarged. Niagara Limestone (Wenlock), Canada. (Original.) represented. We have no trace of the Holothurians or Sea-cucumbers; but this is not surprising, as the record of the past is throughout almost silent as to the former existence of these soft-bodied creatures, the scattered plates and spicules in their skin offering a very uncertain chance of preservation in the fossil condition. The Sea-urchins (Echinoids) are said to be represented by examples of the old genus Palœchinus. The Star-fishes (Asteroids) and the Brittle-stars (Ophiuroids) are, comparatively speaking, largely represented; the former by species of Palasterina (fig. 60), Palœaster (fig. 60), Palœocoma (fig. 60), Petraster, Glyptaster, and Lepidaster—and the latter by species of Protaster (fig. 61), Palœodiscus, Acroura, and Eucladia. The singular Cystideans, or "Globe Crinoids," with their globular or ovate, tesselated bodies (fig. 46, A, C, D,), are also not uncommon in the Upper Silurian; and if they do not become finally extinct here, they certainly survive the close of this period by but a very brief time. By far the most important, however, of the Upper Silurian Echinodenns, are the Sea-lilies or Crinoids. The limestones of this period are often largely composed of the fragmentary columns and detached plates of these creatures, and some of them (such as the Wenlock Limestone of Dudley) have yielded
Fig. 60.—Upper Silurian Star-fishes. 1, Palasterina primœva, Lower Ludlow; 2, Paloeaster Ruthveni, Lower Ludlow; 3, Palœocoma Colvini, Lower Ludlow. (After Salter.) perhaps the most exquisitely-preserved examples of this group with which we are as yet acquainted. However varied in their forms, these beautiful organisms consist of a globular, ovate, or
Fig. 61.—A, Protaster Sedgwickii, showing the disc and bases of the arms; B, Portion of an arm, greatly enlarged. Lower Ludlow. (After Salter.) pear-shaped body (the "calyx"), supported upon a longer or shorter jointed stem (or "column"). The body is covered externally with an armour of closely-fitting calcareous plates (fig. 62), and its upper surface is protected by similar but smaller plates more loosely connected by a leathery integument. From the upper surface of the body, round its margin, springs a series of longer or shorter flexible processes, composed of innumerable calcareous joints or pieces, movably united with one another. The arms are typically five in number; but they generally subdivide at least once, sometimes twice, and they are furnished with similar but
Fig. 62.—Upper Silurian Crinoids. a, Calyx and arms of Eucalyptocrinus polydactylus, Wenlock Limestone; b, Ichthyocrinus lœvis, Niagara Limestone, America; c, Taxocrinus tuberculatus, Wenlock Limestone. (After M'Coy and Hall.) more slender lateral branches or "pinnules," thus giving rise to a crown of delicate feathery plumes. The "column" is the stem by which the animal is attached permanently to the bottom of the sea; and it is composed of numerous separate plates, so jointed together that whilst the amount of movement between any two pieces must be very limited, the entire column acquires more or less flexibility, allowing the organism as a whole to wave backwards and forwards on its stalk. Into the exquisite minutiœ of structure by which the innumerable parts entering into the composition of a single Crinoid are adapted for their proper purposes in the economy of the animal, it is impossible to enter here. No period, as before said, has yielded examples of greater beauty than the Upper Silurian, the principal genera represented being Cyathocrinus, Platycrinus, Marsupiocrinus, Taxocrinus, Eucalyptocrinus, Ichthyocrinus, Mariacrinus, Periechocrinus, Glyptocrinus, Crotalocrinus, and Edriocrinus.
The tracks and burrows of Annelides are as abundant in the Upper Silurian strata as in older deposits, and have just as commonly been regarded as plants. The most abundant forms are the cylindrical, twisted bodies (Planolites), which are so frequently found on the surfaces of sandy beds, and which have been described as the stems of sea-weeds. These fossils (fig. 63), however, can be nothing more, in most
Fig. 63.—Planolites vulgaris, the filled-up burrows of a marine worm. Upper Silurian (Clinton Group), Canada. (Original.) cases, than the filled-up burrows of marine worms resembling the living Lob-worms. There are also various remains which belong to the group of the tube-inhabiting Annelides (Tubicola). Of this nature are the tubes of Serpulites and Cornultites, and the little spiral discs of Spirorbis Lewisii.
Amongst the Articulates, we still meet only with the remains of Crustaceans. Besides the little bivalved Ostracoda—which here are occasionally found of the size of beans—and various Phyllopods of different kinds, we have an abundance of Trilobites. These last-mentioned ancient types, however, are now beginning to show signs of decadence; and though still individually numerous, there is a great diminution in the number of generic types. Many of the old genera, which flourished so abundantly in Lower Silurian seas, have now died out; and the group is represented chiefly by species of Cheirurus, Encrinurus, Harpes, Proetus, Lichas, Acidaspis, Illœnus, Calymene, Homalonotus, and Phacops—the last of these, one of the highest and most beautiful of the groups of Trilobites, attaining here its maximum of development. In the annexed illustration (fig. 64) some of the characteristic Upper Silurian Trilobites are
Fig. 64.—Upper Silurian Trilobites. a, Cheirurus bimucronatus, Wenlock and Caradoc; b, Phacops longicaudatus, Wenlock, Britain, and America; c, Phacops Downingiœ, Wenlock and Ludlow; d, Harpes ungula, Upper Silurian, Bohemia. (After Salter and Barrande.) represented—all, however, belonging to genera which have their commencement in the Lower Silurian period. In addition to the above, the Ludlow rocks of Britain and the Lower Helderberg beds of North America have yielded the remains of certain singular Crustaceans belonging to the extinct order of the Eurypterida. Some of these wonderful forms are not remarkable for their size; but others, such as Pterygotus Anglicus (fig. 65), attain a length of six feet or more, and may fairly be considered as the giants of their class. The Eurypterids are most nearly allied to the existing King-crabs (Limuli), and have the anterior end of the body covered with a great head-shield, carrying two pairs of eyes, the one simple and the other compound. The feelers are converted into pincers, whilst the last pair of limbs have their bases covered with spiny teeth so as to act as jaws, and are flattened and widened out towards their extremities so as to officiate as swimming-paddles. The hinder extremity of the body is composed of thirteen rings, which have no legs attached to them; and the last segment of the tail is either a flattened plate or a narrow, sword-shaped spine. Fragments of the skeleton are easily recognised by the peculiar scale-like markings with
Fig. 65.—Pterygotus Anglicus, viewed from the under side, reduced in size, and restored. c c, The feelers (antennæ), terminating in nipping-claws; o o, Eyes; m m, Three pairs of jointed limbs, with pointed extremities; n n, Swimming-paddles, the bases of which are spiny and act as jaws. Upper Silurian, Lanarkshire. (After Henry Woodward.) which the surface is adorned, and which look not at all unlike the scales of a fish. The most famous locality for these great Crustaceans is Lesmahagow, in Lanarkshire, where many different species have been found. The true King-crabs (Limuli) of existing seas also appear to have been represented by at least one form (Neolimulus) in the Upper Silurian.
Coming to the Mollusca, we note the occurrence of the same great groups as in the Lower Silurian. Amongst the Sea-mosses (Polyzoa), we have the ancient Lace-corals (Fenestella and Retepora), with the nearly-allied Glauconome, and species of Ptilodictya (fig. 66); whilst many forms often referred here may probably have to be transferred to the Corals, just as some so-called Corals will ultimately be removed to the present group.
The Brachiopods continued to flourish during the Upper Silurian Period in immense numbers and under a greatly increased variety of forms. The three prominent Lower Silurian genera Orthis, Strophomena, and Leptœna are still well represented, though they have lost their former preeminence. Amongst the numerous types which have now come upon the scene for the first time, or which have now a special development, are Spirifera and Pentamerus. In the first of these (fig. 69. b, c), one of the valves of the shell (the dorsal) is furnished in its interior with a pair of great calcareous spires, which served for the support of the long and fringed fleshy processes or "arms" which were attached to the sides of the mouth.[16] In the genus Pentamerus (fig. 70) the shell is curiously subdivided in its interior by calcareous plates. The Pentameri commenced their existence at the very close of the Lower Silurian (Llandovery), and
Fig. 66.—Upper Silurian Polyzoa. 1, Fan-shaped frond of Rhinopora verrucosa; 1a, Portion of the surface of the same, enlarged; 2 and 2a, Phœnopora ensiformis, of the natural size and enlarged; 3 and 3a, Helopora fragilis, of the natural size and enlarged; 4 and 4a, Ptilodictya raripora, of the natural size and enlarged. The specimens are all from the Clinton Formation (May Hill Group) of Canada. (Original.) survived to the close of the Upper Silurian; but they are specially characteristic of the May Hill and Wenlock groups, both in Britain and in other regions. One species, Pentamerus galeatus, is common to Sweden, Britain, and America. Amongst the remaining Upper Silurian Brachiopods are the extraordinary Trimerellids; the old and at the same time modern Lingulœ, Discinœ, and Craniœ; together with many species of Atrypa (fig. 68, e),
Fig. 68.—Upper Silurian Brachiopods. a a', Leptocœlia plano-convexa, Clinton Group, America; b b', Rhynchonella neglecta, Clinton Group, America; c, Rhynchonella cuneata, Niagara Group, America, and Wenlock Group, Britain; d d', Orthis elelgantula, Llandeilo to Ludlow, America and Europe; e e', Atrypa hemispherica, Clinton Group, America, and Llandovery and May Hill Groups, Britain; f f', Atrypa congesta, Clinton Group, America; g g', Orthis Davidsoni, Clinton Group, America. (After Hall, Billings, and the Author.) Leptocœlia (fig. 68, a), Rhynchonella (fig. 68, b, c), Meristella (fig. 69, a, e, f), Athyris, Retzia, Chonetes, &c.
[Footnote 16: In all the Lamp-shells the mouth is provided with two long fleshy organs, which carry delicate filaments on their sides, and which are usually coiled into a spiral. These organs are known as the "arms," and it is from their presence that the name of "Brachiopoda" is derived (Gr. brachion, arm; podes, feet). In some cases the arms are merely coiled away within the shell, without any support; but in other cases they are carried upon a more or less elaborate shelly loop, often spoken of as the "carriage-spring apparatus." In the Spirifers, and in other ancient genera, this apparatus is coiled up into a complicated spiral (fig. 67). It is these "arms," with or without
Fig. 67.—Spirifera hysterica. The right-hand figure shows the interior of the dorsal valve with the calcareous spires for the support of the arms. the supporting loops or spires, which serve as one of the special characters distinguishing the Brachiopods from the true Bivalves (Lamellibranchiata).]
| Fig. 69.—a a', Meristella intermedia, Niagara Group, America; b, Spirifera Niagarensis, Niagara Group, America; c c', Spirifera crispa, May Hill to Ludlow, Britain, and Niagara Group, America; d, Strophomena (Streptorhynchus) subplana, Niagara Group, America; e, Meristella naviformis, Niagara Group, America; f, Meristella cylindrica, Niagara Group, America. (After Hall, Billings, and the Author.) |
The higher groups of the Mollusca are also largely represented in the Upper Silurian. Apart from some singular types, such as the huge and thick-shelled Megalomi of the American Wenlock formation, the Bivalves (Lamellibranchiata) present little of
Fig. 70.—Pentamerus Knightii. Wenlock and Ludlow. The right-hand figure shows the internal partitions of the shell. special interest; for though sufficiently numerous, they are rarely well preserved, and their true affinities are often uncertain. Amongst the most characteristic genera of this period may be mentioned Cardiola (fig. 71, A and C) and Pterinea
Fig. 71.—Upper Silurian Bivalves. A, Cardiola interrupta, Wenlock and Ludlow; B, Pterinea subfalcata, Wenlock; C, Cardiola fibrosa, Ludlow. (After Salter and M'Coy.) (fig. 71, B), though the latter survives to a much later date. The Univalves (Gasteropoda) are very numerous, and a few characteristic forms are here figured (fig. 72). Of these, no genus is perhaps more characteristic than Euomphalus (fig. 72, b), with its flat discoidal shell, coiled up into an oblique spiral, and deeply hollowed out on one side; but examples of this group are both of older and of more modern date. Another very extensive genus, especially in America, is Platyceras (fig. 72, a and f), with its thin fragile shell—often hardly coiled up at all—its minute spire, and its widely-expanded, often sinuated mouth. The British Acroculiœ should probably be placed here, and the group has with reason been regarded as allied to the Violet-snails (Ianthina) of the open Atlantic. The species of Platyostoma (fig. 72, h) also belong to the same family; and the entire group is continued throughout the Devonian into the Carboniferous. Amongst other well-known Upper Silurian Gasteropods are species of the genera Holopea (fig. 72, g), Holopella (fig. 72. e),
Fig. 72.—Upper Silurian Gasteropods. a, Platyceras ventricosum, Lower Helderberg, America; b, Euomphalus discors, Wenlock, Britain; c, Holopella obsoleta Ludlow, Britain; d, Platyschisma helicites, Upper Ludlow, Britain; e, Holopella gracilior, Wenlock, Britain; f, Platyceras multisinuatum, Lower Helderberg, America; g, Holopea subconica, Lower Helderberg, America; h, h', Platyostoma Niagarense, Niagara Group, America. (After Hall, M'Coy, and Salter.) Platyschisma (fig. 72, d), Cyclonema, Pleurotomaria, Murchisonia, Trochonema, &c. The oceanic
Fig. 73.—Tentaculites ornatus. Upper Silurian of Europe and North America. Univalves (Heteropods) are represented mainly by species of Bellerophon; and the Winged Snails, or Pteropods, can still boast of the gigantic Thecœ and Conulariœ, which characterise yet older deposits. The commonest genus of Pteropoda, however, is Tentaculites (fig. 73), which clearly belongs here, though it has commonly been regarded as the tube of an Annelide. The shell in this group is a conical tube, usually adorned with prominent transverse rings, and often with finer transverse or longitudinal striæ as well; and many beds of the Upper Silurian exhibit myriads of such tubes scattered promiscuously over their surfaces.
The last and highest group of the Mollusca—that of the Cephalopoda—is still represented only by Tetrabranchiate forms; but the abundance and variety of these is almost beyond belief. Many hundreds of different species are known, chiefly belonging to the straight Orthoceratites, but the slightly-curved Cyrtoceras is only little less common. There are also numerous forms of the genera Phragmoceras, Ascoceras, Gyroteras, Lituites, and Nautilus. Here, also, are the first-known species of the genus Goniatites—a group which attains considerable importance in later deposits, and which is to be regarded as the precursor of the Ammonites of the Secondary period.
Finally, we find ourselves for the first time called upon to consider the remains of undoubted vertebrate animals, in the
Fig. 74.—Head-shield of Pteraspis Banksii, Ludlow rocks. (After Murchison.) form of Fishes. The oldest of these remains, so far as yet known, are found in the Lower Ludlow rocks, and they consist of the bony head-shields or bucklers of certain singular armoured fishes belonging to the group of the Ganoids, represented at the present day by the Sturgeons, the Gar-pikes of North America, and a few other less familiar forms. The principal Upper Silurian genus of these is Pteraspis, and the annexed illustration (fig. 74) will give some idea of the extraordinary form of the shield covering the head in these ancient fishes. The remarkable stratum near the top of the Ludlow formation known as the "bone-bed" has also yielded the remains of shark-like fishes. Some of these, for which the name of Onchus has been proposed, are in the form of compressed, slightly-curved spines (fig. 75, A), which would appear to be of the nature of the strong defensive spines implanted in front of certain of the fins in many living fishes. Besides these, have been found fragments of prickly skin
Fig. 75.—A, Spine of Onchus tenuistriatus; B, Shagreen-scales of Thelodus. Both from the "bone-bed" of the Upper Ludlow rocks. (After Murchison.) or shagreen (Sphagodus), along with minute cushion-shaped bodies (Thelodus, fig. 75, B), which are doubtless the bony scales of some fish resembling the modern Dog-fishes. As the above mentioned remains belong to two distinct, and at the same time highly-organised, groups of the fishes, it is hardly likely that we are really presented here with the first examples of this great class. On the contrary, whether the so-called "Conodonts" should prove to be the teeth of fishes or not, we are justified in expecting that unequivocal remains of this group of animals will still be found in the Lower Silurian. It is interesting, also, to note that the first appearance of fishes—the lowest class of vertebrate animals—so far as known to us at present, does not take place until after all the great sub-kingdoms of invertebrates have been long in existence; and there is no reason for thinking that future discoveries will materially affect the relative order of succession thus indicated.
LITERATURE.
From the vast and daily-increasing mass of Silurian literature, it is impossible to do more than select a small number of works which have a classical and historical interest to the English-speaking geologist, or which embody researches on special groups of Silurian animals—anything like an enumeration of all the works and papers on this subject being wholly out of the question. Apart, therefore, from numerous and in many cases extremely important memoirs, by various well-known observers, both at home and abroad, the following are some of the more weighty works to which the student may refer in investigating the physical characters and succession of the Silurian strata and their fossil contents:—