FOSSIL LYCOPODIALES.

Isoetaceae

The geological history of this division of the Pteridophyta is exceedingly meagre, a fact all the more regrettable as it is by no means improbable that in the surviving genus Isoetes we have an isolated type possibly of considerable antiquity and closely akin to such extinct genera as Pleuromeia and Sigillaria. If Saporta’s Lower Cretaceous species Isoetes Choffati[146], or more appropriately Isoetites Choffati, is correctly determined, it is the oldest fossil member of the family and indeed the most satisfactory among the more than doubtful species described as extinct forms of Isoetes.

Isoetites.

The generic name Isoetites was first used by Münster[147] in the description of a specimen, from the Jurassic lithographic slates of Solenhofen in Bavaria, which he named Isoetites crociformis. The specific name was chosen to express a resemblance of the tuberous appearance of the lower part of the imperfectly preserved and indeterminable fossil to a Crocus corm.

Impressions of Isoetes-like leaves from the Inferior Oolite of Yorkshire figured by Phillips[148] and afterwards by Lindley[149] as Solenites Murrayana were compared by the latter author with Isoetes and Pilularia, but these leaves are now generally assigned to Heer’s gymnospermous genus Czekanowskia. An examination of the structure of the epidermal cells of these Jurassic impressions convinced me that they resemble recent coniferous needles more closely than the leaves of any Pteridophyte. The genus Czekanowskia[150] is recognised by several authors as a probable member of the Ginkgoales.

Isoetites Choffati. Saporta.

The late Marquis of Saporta founded this species on two sets of impressions from the Urgonian (Lower Cretaceous) of Portugal which, though not found in actual organic connexion, may possibly be portions of the same plant. Small relatively broad tuberous bodies reaching a breadth of 1 cm. are compared with the short and broad stem of Isoetes, which they resemble in bearing numerous appendages radiating from the surface like the roots of the recent species; on the exposed face of the stem occur scattered circular scars representing the position of roots which were detached before fossilisation. Other impressions are identified as the basal portions of sporophylls bearing sporangia: these suggest the expanded base of the fertile leaves of Isoetes with vertically elongated sporangia, some of which have a smooth surface while in others traces of internal structure are exposed; the interior consists of an irregular network with depressions containing carbonised remains of spores.

While recognising a general resemblance to the sporophylls of Isoetes, certain differences are obvious: there is no ligule in the fossil leaves nor are there any distinct traces of vascular strands such as occur in the leaves of recent species. The form of the sporangium, more elongated than in the majority of recent forms, is compared by Saporta with that in a south European species Isoetes setacea Spr.

Such evidence as we have lends support to the inclusion of these Portuguese fossils in the genus Isoetites, but apart from the fact that we have no proof of any connexion between the stems and supposed sporophylls, the resemblance of the latter to those of Isoetes is, perhaps, hardly sufficient to satisfy all reasonable scepticism.

The generic name Isoetopsis was used by Saporta as more appropriate than Isoetes for some Eocene fossils from Aix-en-Provence which are too doubtful to rank as trustworthy evidence of the existence of the recent genus. The species, Isoetopsis subaphylla[151] is founded on impressions of small scales, 4 mm. long, bearing circular bodies which are compared with sporangia or spores.

Other records of fossils referred to Isoetes need not be described as they have no claim to be regarded as contributions towards the past history of the genus. Heer’s Miocene species Isoetites Scheuzeri and I. Braunii Unger[152] from Switzerland are based on unsatisfactory material and are of no importance.

Pleuromeia.

The generic name Pleuromeia, was suggested by Corda[153] for a fossil from the Bunter Sandstone, the original description of which was based by Münster[154] on a specimen discovered in a split stone from the tower of Magdeburg Cathedral.

The majority of the specimens have been obtained from the neighbourhood of Bernburg, but a few examples are recorded from Commern and other German localities: all are now included under the name Pleuromeia Sternbergi. Germar, who published one of the earlier accounts of the species, states that Corda dissented from Münster’s choice of the name Sigillaria and proposed the new generic title Pleuromeia. One of the best descriptions of the genus we owe to Solms-Laubach[155] whose paper contains references to earlier writers. Illustrations have been published by Münster, Germar[156], Bischof[157], Solms-Laubach and Potonié[158].

Pleuromeia Sternbergi. (Münster.)
[Fig. 134].

Pleuromeia Sternbergi is represented by casts of vegetative and fertile axes, but the preservation of the latter is not sufficiently good to enable us to draw any very definite conclusions as to the nature of the reproductive organs. Casts of the stems reach a length of about 1 metre and a diameter of 5–6 cm., or in some cases 10 cm.; all of them are in a more or less decorticated state, the degree of decortication being responsible for differences in the external features which led Spieker[160] to adopt more than one specific name.

[Fig. 134], A, represents a sketch, made some years ago, of a specimen in the Breslau Museum which contains several examples of this species, among others those described by Germar in 1852. The cylindrical cast (38 cm. long by 12 cm. in circumference), which has been slightly squeezed towards the upper end, bears spirally arranged imperfectly preserved leaf-scars and the lower end shows the truncated base of one of the short Stigmaria-like arms characteristic of the plant. As shown clearly in a specimen originally figured by Bischof and more recently by Potonié[161], the stem-base is divided by a double dichotomy into four short and broad lobes with blunt apices and bent upwards like the arms of a grappling iron ([fig. 134], D). The surface of this basal region is characterised by numerous circular scars ([fig. 134], D; 4 scars enlarged) in the form of slightly projecting areas with a depression in the centre of each. These are undoubtedly the scars of rootlets, remains of which are occasionally seen radiating through the surrounding rock. As seen in [fig. 134], D, a, the fractured surface of a basal area may reveal the existence of an axial vascular cylinder giving off slender branches to the rootlets.

Fig. 134. Pleuromeia Sternbergi.

The bulbous enlargement at the base of the Brown seaweed Laminaria bulbosa Lam.[162] simulates the swollen base of Pleuromeia; but a confusion between these two plants is hardly likely to occur. Above the Stigmaria-like base the gradually tapered axis, in the less decorticated specimens, bears spirally disposed transversely elongated areas consisting of two triangular scars between which is the point of exit of a leaf-trace. The form of the leaf-scars is best seen on the face of a mould figured by Solms-Laubach ([fig. 134], C): in this case the two triangular areas appear as slight projections separated by a narrow groove marking the position of the vascular bundle of the leaf. The curved lines above and below the leaf-scar probably mark the boundary of the leaf-base. The two triangular scars are compared by Solms-Laubach and by Potonié with the parichnos-scars of Sigillaria and Lepidodendron (cf. [fig. 146], C), but the large size of the Pleuromeia scars constitutes an obvious difference though possibly not a distinction of importance.

The occurrence of a vertical canal filled with carbonaceous material in some of the stems throws light on the internal structure: the canal, which is described by Solms-Laubach as having a stellate outline in transverse section recalls the narrow central cylinder of a Lepidodendron stem, and this comparison is strengthened by the presence of obliquely ascending grooves which represent leaf-traces passing through the cortex. In specimens which have lost more of the cortical tissues the surface is characterised by spirally disposed, discontinuous vertical grooves representing portions of leaf-traces precisely as they appear in similar casts of Lepidodendron. There is no direct evidence of the existence of secondary wood in the stem, but, as Potonié has pointed out, the greater transverse elongation of the leaf-scars in the lower part of a cast ([fig. 134], A) points to the production of some secondary tissue either in the vascular cylinder or cortex, or possibly in both regions.

In some specimens of Pleuromeia the upper portion is clothed with crowded and imbricate sporophylls which reach a length of 2·5 cm., a maximum breadth of 2·7 cm., and a thickness of 1 mm. Each sporophyll has a thin wing-like border, and on the lower face are several parallel lines. Solms-Laubach describes the sporangium or ovule as attached to the lower surface of the sporophyll and this opinion has been confirmed by Fitting[163] who has also brought forward satisfactory evidence in favour of the sporangial nature of the reproductive organs. Fitting found numerous spores in the Bunter Sandstone near Halle; these are flattened circular bodies 0·5–0·7 mm. in diameter with a granulated surface and the three converging lines characteristic of spores produced in tetrads. The comparison made by this author between the sporophylls of Pleuromeia, which bore the sporangia on the lower surface instead of on the upper as in other lycopodiaceous plants, and the pollen-sacs of Conifers, is worthy of note in reference to the possible relationship between Conifers and Lycopods.

A comparison of the Isoetes stem represented in [fig. 132], A, with the base of a Pleuromeia shows a striking similarity, but, as Fitting points out, the Stigmaria-like arms of the fossil contained a vascular cylinder whereas the blunt lobes of Isoetes consist exclusively of cortical tissue, the roots being given off from the grooves between the lobes of the tuberous stem.

The position of Pleuromeia must for the present be left an open question; it is, however, clear that the plant bears a close resemblance in the form of its base to the Stigmarian branches of Lepidodendron and Sigillaria. The vegetative shoot appears to be constructed on a plan similar to that of these two Palaeozoic genera, but the strobilus is of a different type. It would seem probable that Pleuromeia may be closely allied to Isoetes and to the arborescent Lycopods of Palaeozoic floras. It is not improbably a link in a chain of types which includes Sigillaria on the one hand and Isoetes on the other.

It is not improbable that a specimen from the Lower Bunter of Commern which Blanckenhorn made the type of a new species, Sigillaria oculina ([fig. 134], B) is specifically identical with Pleuromeia Sternbergi. An examination of a cast of the type-specimen in the Berlin Bergakademie led me to regard the fossil with some hesitation as a true Sigillaria, but a more extended knowledge of Pleuromeia lends support to the view adopted by Potonié[164] that Blanckenhorn’s plant is not genetically distinct from Pleuromeia Sternbergi. The resemblance between Sigillaria oculina and some of the Palaeozoic species of Sigillaria emphasised by Weiss[165] has given rise to the belief that the genus Sigillaria persisted into the Triassic era; it is, however, highly probable that the Bunter specimen has no claim to the generic name under which it has hither to been known.

The Bunter Sandstone in which Pleuromeia is the sole representative of plant-life, at least in certain localities, is usually considered to be a desert formation. We may not be far wrong in accepting Fitting’s suggestion that in this isolated species we have a relic of the sparse vegetation which was able to exist where the presence of lakes added a touch of life to the deadness of the Triassic desert.

Pleuromeia is recorded by Fliche as a rare fossil in the Middle Trias of France in the neighbourhood of Lunéville[166].

Herbaceous fossil species of Lycopodiales.

The history of our knowledge of fossil representatives of the Lycopodiales, as also of the Equisetales, affords a striking illustration of the danger of attempting to found a classification on such differences as are expressed by the terms herbaceous and arborescent in the sense in which they are usually employed. As we have seen[167], the presence of secondary wood in stems of the Palaeozoic plant now known as Calamites led so competent a botanist as Adolphe Brongniart to recognise a distinct generic type Calamodendron, which he placed in the Gymnosperms, reserving the designation Calamities for species in which no indication of secondary thickening had been found.

Similarly, the genus Sigillaria was regarded as a Gymnosperm because it was believed to be distinguished from Lepidodendron by the power of forming secondary vascular tissues; the latter genus, originally thought to be always herbaceous, was classed with the Pteridophytes. At the time when this unnatural separation was made between stems with secondary wood and those in which no secondary wood was known to exist, botanists were not aware of the occurrence of any recent Pteridophyte which shared with the higher plants the power of secondary growth in thickness provided by means of a meristematic zone. It is true that the presence or absence of a cambium does not in practice always coincide with the division into herbaceous and arborescent plants: no one would speak of a Date-Palm as a herbaceous plant despite the absence of secondary wood.

The danger which should be borne in mind, in adopting as a matter of convenience the term herbaceous as a sectional heading, is that it should not be taken to imply a complete inability of the so-called herbaceous types to make secondary additions to their conducting tissues. The specimens on which the species of Lycopodites and Selaginellites, (genera which may be designated herbaceous,) are founded are preserved as impressions and not as petrifications; we can, therefore, base definitions only on habit and on such features as are shown by fertile leaves and sporangia. We are fully justified in concluding from evidence adduced by Goldenberg more than fifty years ago and from similar evidence brought to light by more recent researches, that there existed in the Palaeozoic era lycopodiaceous species in close agreement in their herbaceous habit with the lycopods of present-day floras. It has been suggested[168] that the direct ancestors of the genera Lycopodium and Selaginella are represented by the species of Lycopodites and Selaginellites rather than by Lepidodendron and Sigillaria, the arborescent habit of which has been rendered familiar by the numerous attempts to furnish pictorial reproductions of a Palaeozoic forest. Until we are able to subject the species classed as herbaceous to microscopical examination we cannot make any positive statement as to the correctness of this view, but such facts as we possess lead us to regard the suggestion as resting on a sound basis.

Palaeobotanical literature abounds in records of species of Lycopodites, Lycopodium, Selaginella and Selaginites, which have been so named in the belief that their vegetative shoots bear a greater resemblance to those of recent lycopodiaceous plants than to the foliage shoots of Lepidodendron. Many of these records are valueless: Lepidodendra, twigs of Bothrodendron[169] species of conifers, fern rhizomes, and Aphlebiae[170] have masqueraded as herbaceous lycopods. It is obvious that an attempt to identify fossils presenting a general agreement in habit and leaf-form with recent species of lycopods must be attended with considerable risk of error. Recent Conifers include several species the smaller branches of which simulate the leafy shoots of certain species of Lycopodium and Selaginella, and it is not surprising to find that this similarity has been responsible for many false determinations. Among Mosses and the larger foliose Liverworts there are species which in the condition of imperfectly preserved impressions, might easily be mistaken for lycopodiaceous shoots: an equally close resemblance is apparent in the case of some flowering plants, such as New Zealand species of Veronica, Tafalla graveolens (a Composite), Lavoisiera lycopodiodes Gard.[171] (a species of Melastomaceae), all of which have the habit of Cupressineae among the conifers as well as of certain lycopodiaceous plants. It may be impossible to decide whether fossil impressions of branches, which are presumably lycopodiaceous, bear two kinds of leaves[172] like the great majority of recent species of Selaginella. Selaginella grandis, if seen from the under surface, would appear to have two rows of leaves only and might be confused with a small twig of such a conifer as Dacrydium Kirkii, a New Zealand species.

The New Zealand conifers Dacrydium cupressinum Soland. and Podocarpus dacrydioides Rich. closely simulate species of Selaginellites and Lycopodites: in the British Museum a specimen of the latter species bears a label describing it as Lycopodium arboreum (Sir Joseph Hooker and Dr Solander; 1769). The twigs of the Tasmanian conifer Microcachyrs tetragona Hook. f. are very similar in habit to shoots of the recent Lycopodium tetragonum ([fig. 121], C).

In the description of examples of Lycopodites and Selaginellites I have confined myself to such as appear to be above suspicion either because of the presence of spore-bearing organs or, in a few cases, because the specimens of sterile shoots are sufficiently large to show the form of branching in addition to the texture of the leaves. The two generic names Lycopodites and Selaginellites are employed for fossil species which there are substantial grounds for regarding as representatives of Lycopodium and Selaginella. The designation Selaginellites is adopted only for species which afford evidence of heterospory; the name Lycopodites, on the other hand, is used in a comprehensive sense to include all forms—whether homophyllous or heterophyllous—which are not known to be heterosporous. This restricted use of the generic name Selaginellites is advocated by Zeiller[173], who instituted the genus, and by Halle[174] in his recent paper on herbaceous lycopods.

Lycopodites.

The generic term Lycopodites was used by Brongniart in 1822[175] in describing some Tertiary examples of slender axes clothed with small scale-like leaves which he named Lycopodites squamatus. These are fragments of coniferous shoots. In the Prodrome d’une histoire des végétaux fossiles[176] Brongniart included several Palaeozoic and Jurassic species in Lycopodites and instituted a new genus Selaginites, expressing a doubt as to the wisdom of attempting to draw a generic distinction between the two sets of species. In a later work[177] he recognised only one undoubted species, Lycopodites falcatus. The first satisfactory account of fossils referred to Lycopodites is by Goldenberg[178] who gave the following definition of the genus:—“Branches with leaves spirally disposed or in whorls. Sporangia in the axil of foliage leaves or borne in terminal strobili.”

It was suggested by Lesquereux[179] that Goldenberg’s definition, which was intended to apply to herbaceous species, should be extended so as to include forms with woody stems but which do not in all respects agree with Lepidodendron. Kidston[180] subsequently adopted Lesquereux’s modification of Goldenberg’s definition. We cannot draw any well-defined line between impressions of herbaceous forms and those of small arborescent species. We use the name Lycopodites for such plants as appear to agree in habit with recent species of Lycopodium and Selaginella and which, so far as we know, were not heterosporous: it is highly probable that some of the species so named had the power of producing secondary wood, a power possessed by some recent Pteridophytes which never attain the dimensions of arborescent plants.

It has been shown by Halle[181], who has re-examined several of Goldenberg’s specimens which have been acquired by the Stockholm Palaeobotanical Museum, that some of his species of Lycopodites are heterosporous and therefore referable to Zeiller’s genus Selaginellites.

In 1869 Renault described two species of supposed Palaeozoic Lycopods as Lycopodium punctatum and L. Renaultii[182], the latter name having been suggested by Brongniart to whom specimens were submitted. These species were afterwards recognised by their author as wrongly named and were transferred to the genus Heterangium[183], a determination which is probably correct; it is at least certain that the use of the name Lycopodium cannot be upheld.

We have unfortunately to rely on specimens without petrified tissues for our information in regard to the history of Lycopodites and Selaginellites. Among the older fossils referred to Lycopodites are specimens from Lower Carboniferous rocks at Shap in Westmoreland which Kidston originally described as Lycopodites Vanuxemi[184], identifying them with Goeppert’s Sigillaria Vanuxemi[185] founded on German material. In a later paper Kidston transferred the British specimens of vegetative shoots to a new genus Archaeosigillaria[186].

Lycopodites Stockii Kidston[187].

The plant so named was discovered in Lower Carboniferous strata of Eskdale, Dumfries, Scotland; it is represented by imperfectly preserved shoots bearing a terminal strobilus and was originally described by Kidston as apparently possessing two kinds of foliage leaves borne in whorls. The larger leaves have an ovate cordate lamina with an acuminate apex, while the smaller leaves, which are less distinct, are transversely elongated, and simulate sporangia in appearance. Dr Kidston’s figure of this species has recently been reproduced by Professor Bower[188] who speaks of the supposed smaller leaves as sporangia, a view with which the author of the species agrees. It would appear that this identification is, however, based solely on external resemblance and has not been confirmed by the discovery of any spores. Assuming the sporangial nature of these structures, this Palaeozoic type represents, as Bower points out, a condition similar to that in some recent species of Lycopodium in which sporangia are not confined to a terminal strobilus but occur also in association with ordinary foliage leaves. The strobilus consists of crowded sporophylls which are too imperfect to afford any definite evidence as to their homosporous or heterosporous nature. As Solms-Laubach[189] points out, this type recalls Lycopodium Phlegmaria among recent species.

Lycopodites Reidii Penhallow.

Professor Penhallow[190] instituted this name for a specimen measuring 8 cm. long by 6 mm. in breadth, collected by Mr Reid from the Old Red Sandstone of Caithness, consisting of an axis bearing narrow lanceolate leaves some of which bear sporangia at the base.

Lycopodites Gutbieri Goeppert[191].

1894, Lycopodites elongatus Kidston[192] (not Goldenberg).

The species, figured by Geinitz as Lycopodites Gutbieri[193], from the Coal-Measures of Saxony is probably a true representative of the genus. The Saxon specimens are heterophyllous; the larger lanceolate and slightly falcate leaves arranged in two rows, are 4–5 mm. long while the smaller leaves are one half or one third this size; some of the dichotomously branched shoots terminate in long and narrow strobili not unlike those of Zeiller’s species Selaginellites Suissei[194]. Kidston[195] has included under this specific name some fragments collected by Hemingway from the Upper Coal-Measures of Radstock, Somersetshire, but as only one form of leaf is seen the reasons for adopting Goeppert’s designation are perhaps hardly adequate.

Lycopodites ciliatus Kidston[196].

Under this name Kidston describes a small specimen, obtained by Hemingway from the Middle Coal-Measures of Barnsley in Yorkshire, consisting of a slender forked axis bearing oval-acuminate leaves approximately 5 mm. long with a finely ciliate margin. Associated with the leaves were found spores which Kidston regards as megaspores.

Lycopodites macrophyllus Goldenberg[197].

This species, originally described by Goldenberg from the Coal-Measures of Saarbrücken has been re-examined by Halle[198] who is unable to confirm Goldenberg’s statement as to heterophylly. The shoots closely resemble Selaginellites primaevus[199] (Gold).

Fig. 135. Selaginellites and Lycopodites. (After Halle.)

  1. Selaginellites primaevus (Gold.). × 10.
  2. Megaspore of Selaginellites elongatus (Gold.). × 50.
  3. Lycopodites Zeilleri Halle. (Nat. size.)
  4. Selaginellites elongatus (Gold.). × 2.
Lycopodites Zeilleri Halle[200]. Fig. 135, C.

Halle has founded this species on specimens, from the Coal-Measures of Zwickau in Saxony, characterised by dimorphic lanceolate leaves in four rows, the larger being 4–6 mm. long: the smaller leaves have a ciliate edge. A comparison is made with the recent species Selaginella arabica Baker, S. revoluta Bak., and S. armata Bak. in which the leaves are described as ciliate. In the absence of sporangia and spores the species is placed in the genus Lycopodites.

Lycopodites lanceolatus (Brodie). Fig. 136.

Fig. 136. Lycopodites lanceolatus (Brodie). (After Miss Sollas. × 40.)
a, Sporangium wall; b, leaf.
c, remains of tubular elements in stem.

Specimens referred to this species were originally recorded by Brodie from Rhaetic rocks in the Severn valley, the name Naiadita being chosen as the result of Lindley’s comparison of the small and delicate leaves with those of recent species of the Monocotyledonous family Naiadaceae. The species may be described as follows:

Plant slender and moss-like in habit. The axis, which is delicate and thread-like, bears numerous linear acuminate or narrow ovate leaves reaching a length of approximately 5 mm. Under a low magnifying power the thin lamina of the leaves is seen to have a superficial layer of polygonal or rectangular cells arranged in parallel series ([fig. 136] b). There is no trace of midrib or stomata. The sporangia are more or less spherical and short-stalked, situated at the base of the foliage leaves and containing numerous tetrads of spores. The spores have a diameter of 0·08 mm.

Buckman founded additional species on differences in the shape of the leaves but, as Miss Sollas has pointed out, such differences as he noticed may be detected on the same axis. It was stated in an earlier chapter[208] that Starkie Gardner, on insufficient evidence, proposed to place Brodie’s plant among the Mosses. The discovery by Mr Wickes of new material at Pylle hill near Bristol afforded an opportunity for a re-examination of the species: this was successfully undertaken by Miss Sollas who was able to dissolve out spores from the matrix by dilute hydrochloric acid, and to recognise the remains of internal structure in the slender axes by exposing successive surfaces with the aid of a hone. It was found that sporangia occurred at the base of some of the leaves containing numerous tetrads of spores, the individual spores having a diameter of 0·08 mm., apparently twice as large as those of any recent species of Lycopodium. Fig. 136 shows a sporangium, a, at the base of a leaf, b. Indications of tubular elements were recognised in the stem and it is noteworthy that although the outlines of epidermal cells on the leaves are well preserved no stomata were found. The leaves of the recent American species Lycopodium alopecuroides Linn. var. aquaticum Spring[209], which lives under water, possess stomata. It is probable that in Lycopodites lanceolatus the leaves had a very thin lamina and may have been similar in structure to those of recent Mosses; the plant possibly lived in very humid situations or grew submerged. Miss Sollas’s investigations afford a satisfactory demonstration of the lycopodiaceous nature of this small Rhaetic species: as I have elsewhere suggested[210], the generic name Lycopodites should be substituted for that of Naiadita. Examples of this species may be seen in the British Museum.

The Rhaetic species from Scania, Lycopodites scanicus Nath.[211] (in litt.), recently re-described by Halle and originally referred by Nathorst to Gleichenia affords another example of the occurrence of a small herbaceous lycopod of Rhaetic age.

Fig. 137. Lycopodites falcatus L. and H. From the Inferior Oolite of Yorkshire. (Nat. size. M.S.)

Lycopodites falcatus Lind. and Hutt. Fig. 137.

In 1822 Young and Bird[215] figured a specimen from the Inferior Oolite rocks of the Yorkshire coast bearing “small round crowded leaves,” which was afterwards described by Lindley from additional material obtained from Cloughton near Scarborough as Lycopodites falcatus. The example represented in [fig. 137] shows the dichotomously branched shoots bearing two rows of broadly falcate leaves. A careful examination of the type-specimen[216] revealed traces of what appeared to be smaller leaves, but there is no satisfactory proof of heterophylly. No sporangia or spores have been found. This British species has been recorded from Lower Jurassic or Rhaetic rocks of Bornholm[217] and a similar though probably not identical type, Lycopodites Victoriae[218], has been recognised in Jurassic strata of Australia (South Gippsland, Victoria). An Indian plant described by Oldham and Morris[219] from the Jurassic flora of the Rajmahal hills as Araucarites (?) gracilis and subsequently transferred by Feistmantel to Schimper’s genus Cheirolepis[220] may be identical with the Yorkshire species. The Jurassic fragments described by Heer from Siberia as Lycopodites tenerrimus[221] may be lycopodiaceous, but they are of no botanical interest.

Other examples of Mesozoic Lycopods have been recorded, but in the absence of well-preserved shoots and sporangia they are noteworthy only as pointing to a wide distribution of Lycopodites in Jurassic and Cretaceous floras[222].

From Tertiary strata species of supposed herbaceous lycopods have been figured by several authors, one of the best of which is Selaginella Berthoudi Lesq.[223] from Tertiary beds in Colorado. This species agrees very closely in the two forms of leaf with Selaginella grandis, but as the specimens are sterile we have not sufficient justification for the employment of the generic name Selaginellites.

Selaginellites.

This generic name has been instituted by Zeiller[224] for specimens from the coal basis of Blanzy (France). It is applied to heterosporous species with the habit of Selaginella: Zeiller preferred the designation Selaginellites to Selaginella on the ground that the type species differs from recent forms in having more than four megaspores in each megasporangium. It is, however, convenient to extend the term to all heterosporous fossil species irrespective of the spore-output.

Selaginellites Suissei Zeiller.

This species was described in Zeiller’s preliminary note[225] as Lycopodites Suissei, but he afterwards transferred it to the genus Selaginellites. In habit the plant bears a close resemblance to Lycopodites macrophyllus of Goldenberg; the shoots, 1–3 mm. thick, are branched in a more or less dichotomous fashion and bear tetrastichous leaves. The larger leaves reach a length of 4–6 mm. and a breadth of 2–3 mm.; the smaller leaves are described as almost invisible, closely applied to the axis, oval-lanceolate and 1–2 mm. long with a breadth of 0·5–0·75 mm. Long and narrow strobili (15 cm. by 8–10 mm.) terminate the fertile branches; these bear crowded sporophylls with a triangular lamina and finely denticulate margin. Oval sporangia were found on the lower sporophylls containing 16–24 spherical megaspores 0·6–0·65 mm. in diameter. The outer membrane of the spore is characterised by fine anastomosing ridges and thin plates radiating from the apex and forming an equatorial collarette. The microspores have a diameter of 40–60μ and the same type of outer membrane as in the megaspores. The megaspores of the recent species Selaginella caulescens, as figured by Bennie and Kidston[226], resemble those of the Palaeozoic type in the presence of an equatorial flange. It is interesting to find that, in spite of the occurrence of 16–24 megaspores in a single sporangium the size of the fossil spores exceeds that of the recent species.

Selaginellites primaevus (Gold.). [Fig. 135], A, [fig. 138].

Fig. 138. Selaginellites primaevus (Gold.). (After Goldenberg.)

In habit this species, first recorded by Goldenberg from the Coal-Measures of Saarbrücken, is similar to S. Suissei Zeill.

The drawing reproduced in [fig. 138] is a copy of that of the type-specimen: another specimen, named by Goldenberg, is figured by Halle in his recently published paper. The leaves appear to be distichous: no smaller leaves have been detected, though Halle is inclined to regard the plant as heterophyllous. The sporophylls, borne in slender terminal strobili, are smaller than the foliage leaves and spirally disposed ([fig. 138]; smaller specimen). Halle succeeded in demonstrating that some of the sporangia contained a single tetrad of spores, each spore having a diameter of 0·4–0·5 mm. No microspores were found, but it is clear that the species was heterosporous and that it agrees with recent species in having only four spores in the megasporangium.

Selaginellites elongatus (Gold.). [Fig. 135], B, D.

The shoots of this species resemble the recent Lycopodium complanatum; they differ from those of Selaginellites primaevus in their long and narrow branches which bear two forms of leaf. The longer leaves, arranged in opposite pairs, are slightly falcate; the smaller leaves are appressed to the axis and have a triangular cordate lamina. Another peculiarity of this species is the occurrence of sporangia in the axil of the foliage leaves, a feature characteristic of the recent Lycopodium Selago. In recent species of Selaginella the sporophylls are always in strobili. No microspores have been found nor the walls of megasporangia, but tetrads of megaspores were isolated by Halle: the spores have three radiating ridges ([fig. 135], B) connected by an equatorial ridge. Halle estimates the number of spores (0·45 mm. in diameter) in a sporangium at 20 to 30. In size as in number the spores exceed those of recent species and agree more nearly with the megaspores of S. Suissei.

It would seem to be a general rule that the spores (megaspores) of the fossil herbaceous species exceeded considerably in dimensions those of recent forms and on the other hand were smaller than those of the Palaeozoic arborescent species.

There can be little doubt that some of the Mesozoic and Tertiary species included under Lycopodites agree more closely with the recent genus Selaginella than with Lycopodium, but this does not constitute an argument of any importance against the restricted use of the designation Selaginellites which we have adopted. From a botanical point of view the various records of Lycopodites and Selaginellites have but a minor importance; they are not sufficiently numerous to throw any light on questions of distribution in former periods, nor is the preservation of the material such as to enable us to compare the fossil with recent types either as regards their anatomy or, except in a few cases, their sporangia and spores. The Palaeozoic species are interesting as revealing less reduction in the number of spores produced in the megasporangia. Among existing Pteridophytes the genus Isoetes agrees more closely than Selaginella, as regards the number of megaspores in each sporangium, with such fossils as Selaginellites Suissei and S. elongatus.

It would seem that in most Palaeozoic species heterospory had not reached the same stage of development as in the recent genus Selaginella in which the megaspores do not exceed four in each sporangium. In Selaginellites primaevus, however, the heterospory appears to be precisely of the same type as in existing species.

Lycostrobus.

The generic name Lycostrobus has recently been instituted by Nathorst[232] for certain specimens of a lycopodiaceous strobilus, from the Rhaetic strata of Scania, which he formerly referred to the genus Androstrobus[233].

Lycostrobus Scotti Nathorst. Fig. 139.

The fossil described under this name is of special interest as affording an example of a Mesozoic lycopodiaceous cone comparable in habit and in size with some of the largest examples of Palaeozoic Lepidostrobi, the cones of Lepidodendron. The Swedish fossil from Upper Rhaetic strata of Helsingborg (Scania) was originally designated Androstrobus Scotti, the generic name being adopted in view of the close resemblance of the form of the strobilus to the male flower of a Cycad. A more complete examination has shown that the bodies, which were thought to be pollen-sacs—though Nathorst recognised certain differences between them and the pollen-sacs of lycopods—are the megaspores of a lycopod. Microspores have also been identified. The axis of the cone has a breadth of 2 cm. with a peduncle which may be naked or provided with a few small scales; the sporophyll region of the axis reached a length of at least 12 cm. The spirally disposed sporophylls terminate in a rhombic distal end which may represent the original termination or they may have been prolonged upwards as free laminae. Each sporophyll bears on its upper face a single large sporangium containing either megaspores or microspores: the megaspores, 0·55–0·60 mm. in diameter, are finely granulate and bear small warty thorns or more slender pointed appendages. The microspores, after treatment with eau de Javelle, were found to measure 36–44μ while others which had been treated with ammonia reached 54μ in diameter. Nathorst describes the microspores as occurring in spherical groups or balls, which it is suggested may be compared with the groups of spores separated by strands of sterile tissue (trabeculae) in the large sporangia of Isoetes (cf. [fig. 133], H). If this comparison is sound it would point to a more complete septation of the sporangium in Lycostrobus than in any recent species of Isoetes. The size of the strobilus would seem to indicate the persistence into the Rhaetic era of an arborescent lycopodiaceous type; but the appearance and manner of preservation of the axis is interpreted by Nathorst as evidence of a herbaceous rather than a woody structure. He is disposed to regard Isoetes as the most nearly allied existing genus.

Fig. 139. Lycostrobus Scotti, Nath. (After Nathorst; ⅘ nat. size.)

The comparison made by Nathorst with Isoetes is based on a resemblance between the spores of the two genera and on the evidence, which is not decisive, of the existence of sterile strands of tissue in the sporangia of Lycostrobus. This similarity is however hardly of sufficient importance to justify the inclusion of the Rhaetic strobilus in the Isoetaceae. In size and in the arrangement and form of the sporophylls the cone presents a much closer resemblance to Lepidodendron than to Isoetes. It is probably advisable to regard this Rhaetic type simply as a lycopodiaceous genus which we are unable, without additional information, to assign to a particular position.

The opinion expressed by Professor Fliche[234] that the plant described by Schimper and Mougeot as Caulopteris tessellata, a supposed tree-fern stem, from Triassic rocks of Lorraine, is more probably a large lycopodiaceous stem, either a Lepidodendron or a new genus, is worthy of note in reference to Nathorst’s account of Lycostrobus.

In habit the fossil strobilus may be compared with the Triassic genus Pleuromeia, but the position of the sporangia on the sporophylls constitutes a well-marked difference. The most important result of Nathorst’s skillful treatment of this interesting fossil by chemical microscopic methods is the demonstration of the existence of a large heterosporous type of lycopodiaceous cone in a Rhaetic flora.

Poecilitostachys.

Under this generic name M. Fliche[235] has briefly described a fertile lycopodiaceous shoot from the Triassic rocks of Epinal in France: the type species Poecilitostachys Hangi consists of a cylindrical axis, 10 cm. × 5 mm., deprived of leaves and terminating in a rounded receptacle bearing a capitulum of bracts or fertile leaves. Detached megasporangia containing small globular bodies found in association with the capitulum are compared with the megasporangia of Isoetes.