Osmundaceae.

From the Culm of Silesia, Stur[761] described impressions of sterile fronds which he named Todea Lipoldi on the ground of the similarity of the finely divided pinnules to those of Todea superba and other filmy species of the genus. The type-specimen of Stur (in the Geological Survey Museum, Vienna) affords no information as to sporangial characters and cannot be accepted as an authentic record of a Lower Carboniferous representative of the family. Another more satisfactory but hardly convincing piece of evidence bearing on the presence of Osmundaceae in pre-Permian floras has been adduced by Renault[762], who described petrified sporangia from the Culm beds of Esnost in France as Todeopsis primaeva ([fig. 256], F). These pyriform sporangia are characterised by the presence of a plate of large cells comparable with the subapical group of “annulus” cells in the sporangia of the recent species ([fig. 221]).

Zeiller[763] has published a figure of some sporangia described by Renault from Autun resembling the Osmundaceous type in having a plate of thick-walled cells instead of a true annulus, but the plate is larger than the group of cells in the recent sporangia, and both sporangia and spores are smaller in the fossil. The sporangia from Carboniferous rocks described by Weiss as Sturiella[764] bear some resemblance to those of recent Osmundaceae, but there is no adequate reason for referring them to this family.

The generic name Pteridotheca is employed by Scott as a convenient designation for unassigned petrified sporangia of Palaeozoic age with an annulus and other characters indicating fern-affinity. In the species P. Butterworthi[765] the sporangia are characterised by a group of large cells suggesting comparison with the annulus, or what represents the annulus, in Osmundaceae and Marattiaceae. Scott has also described a sporangium from the Coal-Measures containing germinating spores[766]; the structure is similar to that of recent Osmundaceous sporangia, and it is interesting to note that germinating spores have been observed in the recent species Todea hymenophylloides[767].

Additional evidence of the same kind is afforded by fertile specimens of a quadripinnate fern with deeply dissected oval-lanceolate pinnules described by Zeiller from the Coal-Measures of Heraclea in Asia Minor as Kidstonia heracleensis[768] (fig. 256, E). Carbonised sporangia were found at the base of narrow lobes of the ultimate segments and, as seen in fig. 256, E, the sporangial wall is distinguished by a plate of larger cells occupying a position like that of the “annulus” of recent Osmundaceae. Zeiller regards the sporangia as intermediate between those of Osmundaceae and Schizaeaceae. From the same locality Zeiller describes another frond bearing somewhat similar sporangia as Sphenopteris (Discopteris) Rallii (fig. 256, D)[769]: the term Discopteris was instituted by Stur for fertile fronds referred by him to the Marattiaceae[770].

It is by no means safe to assume that these and such Upper Carboniferous sporangia as Bower[771] compared with those of Todea were borne on plants possessing the anatomical characters of Osmundaceae rather than those of the extinct Palaeozoic family Botryopterideae. This brings us to the important fact, first pointed out by Renault, that the Botryopterideae are essentially generalised ferns exhibiting many points of contact with the Osmundaceae[772]. It is clear that whether or not we are justified in tracing the Osmundaceae as far back as the Lower Carboniferous period, some of the characteristics of the family were already foreshadowed in rocks of this age.

Through a fortunate accident of preservation, unequivocal evidence of the existence of Osmundaceae in the Palaeozoic era is supplied by the Russian Upper Permian genera Zalesskya and Thamnopteris.

Zalesskya.

This generic title has been instituted by Kidston and Gwynne-Vaughan[773] for two Russian stems of Upper Permian age, one of which was named by Eichwald[774] Chelepteris gracilis, but the probability that the type of the genus Chelepteris is generically distinct from Eichwald’s species necessitated a new designation for the Permian fern.

In habit the stem of Zalesskya resembles that of an Osmunda or a Todea, but it differs in the possession of a stele composed of a continuous cylinder or solid column of xylem surrounded by phloem, and by the differentiation of the xylem into two concentric zones. The leaves are represented by petiole-bases only; the sporangia are unknown. The stem and leaf-base anatomy fully justifies the inclusion of Zalesskya in the Osmundaceae.

Zalesskya gracilis (Eichwald). [Fig. 248].

The type-specimen is a partially decorticated stem, from Upper Permian beds in Russia, provided with a single stele, 13 mm. in diameter, surrounded by a broad thin-walled inner cortex containing numerous leaf-traces and occasional roots: this was doubtless succeeded by a sclerotic outer cortex. In its main features Zalesskya gracilis agrees closely with Z. diploxylon represented in [fig. 249]. The stele consists of a continuous cylinder of xylem exhibiting a fairly distinct differentiation into two zones, (i) a broader outer zone of narrower scalariform tracheae (x ii, [fig. 248]) in which 20 to 25 protoxylem strands (px) occur just within the edge, (ii) an inner zone of broader and shorter tracheae ([fig. 248], x i). The protoxylem elements (px, [fig. 248]) are characterised by a single series of scalariform pits, while the metaxylem elements have multiseriate pits like those on the water-conducting elements of recent Osmundaceae. The tracheae show an interesting histological character in the absence of the middle substance of their walls, a feature recognised by Gwynne-Vaughan[775] in many recent ferns. External to the xylem and separated from it by a parenchymatous sheath is a ring of phloem, ph, composed of large sieve-tubes and parenchyma separated from the inner cortex by a pericycle 4 to 5 layers in breadth. The occurrence of a few sclerotic cells beyond the broad inner cortex points to the former existence of a thick-walled outer cortex. The leaf-traces are given off as mesarch strands from the edge of the xylem; they begin as prominences opposite the protoxylem and become gradually detached as xylem bundles, at first oblong in transverse section, then assuming a slightly crescentic and reniform shape, while the mesarch protoxylem strand takes up an endarch position. As a trace passes further out the curvature increases and the protoxylem strands undergo repeated bifurcation; it assumes in fact the form and general type of structure met with in the leaf-traces of Todea and Osmunda. Numerous diarch roots, given off from the stele at points just below the outgoing leaf-traces, pass outwards in a sinuous horizontal course through the cortex of the stem.

Fig. 248. Zalesskya gracilis (Eich.). Transverse section of part of the stele: ph, phloem; x i, x ii, xylem; px, protoxylem. (After Kidston and Gwynne-Vaughan. × 20.)

Fig. 249. Zalesskya diploxylon. Kidston and Gwynne-Vaughan. Transverse section of stem. ph, phloem. (After Kidston and Gwynne-Vaughan. × 2½.)

In Zalesskya gracilis the xylem cylinder was probably wider in the living plant than in the petrified stem. In Zalesskya diploxylon[776], in all probability from the same Russian locality, there can be little doubt that the xylem was originally solid to the centre ([fig. 249]). In this species also the phloem forms a continuous band (ph, [fig. 249]) consisting of four to six layers of sieve-tubes.

Thamnopteris.

Thamnopteris Schlechtendalii (Eich.). Figs. [250], [312], A, Frontispiece.

In 1849 Brongniart[777] proposed the name Thamnopteris for a species of fern from the Upper Permian of Russia originally described by Eichwald as Anomopteris Schlechtendalii. A new name was employed by Brongniart on the ground that the fossil was not generically identical with the species previously named by him Anomopteris Mougeotii[778]. Eichwald’s specimen has been thoroughly investigated by Kidston and Gwynne-Vaughan[779]. The stem (Frontispiece) agrees in habit with those of Zalesskya and recent Osmundaceae; on the exposed leaf-bases the action of the weather has etched out the horse-shoe form of the vascular strands and laid bare numerous branched roots boring their way through the petiole stumps. The centre of the stem is occupied by a protostele 13 mm. in diameter consisting of solid xylem separated by a parenchymatous sheath from a cylinder of phloem. The xylem is composed mainly of an axial column of short and broad reticulately pitted tracheae ([fig. 250], b and Frontispiece), distinguished from the sharply contrasted peripheral zone of normal scalariform elements, a, by their thinner walls and more irregular shape. The protoxylem, px, is represented by groups of narrower elements rather deeply immersed in the peripheral part of the metaxylem. A many-layered pericycle, per, and traces of an endodermis, en, succeed the phloem, ph, which is characterised by several rows of large contiguous sieve-tubes; beyond the endodermis is a broad thin-walled inner cortex. The leaf-traces arise as in Zalesskya, but the protoxylem in Thamnopteris is at first central; as the trace passes outwards a group of parenchyma appears immediately internal to the protoxylem elements and gradually assumes the form of a bay of thin-walled tissue on the inner concave face of the curved xylem. The next stage is the repeated division of the protoxylem strand until, in the sclerotic outer cortex, the traces acquire the Osmundaceous structure ([fig. 312], A, p. 453). The petiole bases have stipular wings as in Todea and Osmunda.

Fig. 250. Thamnopteris Schlechtendalii (Eich.). Part of stele: a, outer xylem; b, inner xylem. (After Kidston and Gwynne-Vaughan. × 13.)

OSMUNDACEAE

The striking feature exhibited by these Permian plants is the structure of the protostele, which in Thamnopteris and probably in Zalesskya diploxylon consists of solid xylem surrounded by phloem: this may be regarded as the primitive form of the Osmundaceous stele. In Osmunda regalis and in other recent species of the genus the xylem cylinder has the form of a lattice-work; in other words, the departure of each leaf-trace makes a gap in the xylem and the overlapping of the foliar-gaps results in the separation of the xylem into a number of distinct bundles. In Zalesskya gracilis the continuity of the xylem is not broken by overlapping gaps; in this it agrees with Lepidodendron. In Thamnopteris the centre of the stele was occupied by a peculiar form of xylem obviously ill-adapted for conduction, but probably serving for water-storage and comparable with the short and broad tracheae in Megaloxylon[780]. There is clearly a well-marked difference in stelar anatomy between these two Permian genera and Todea and Osmunda: this difference appears less when viewed in the light of the facts revealed by a study of the Jurassic species Osmundites Dunlopi.

Fig. 251. Lonchopteris virginiensis. (After Fontaine. ½ nat. size.)

As possible examples of Triassic Osmundaceae reference may be made to some species included in Stur’s genus Speirocarpus[781]. S. virginiensis was originally described by Fontaine[782] from the Upper Triassic rocks of Virginia as Lonchopteris virginiensis ([fig. 251]) and has recently been figured by Leuthardt[783] from the Keuper of Basel. The sporangia, which are scattered over the lower surface of the pinnules, are described as globose-elliptical and as having a rudimentary apical annulus; no figures have been published. In habit the frond agrees with Todites Williamsoni, but the lateral veins form an anastomosing system like that in the Palaeozoic genus Lonchopteris ([fig. 290], B). There would seem to be an a priori probability of this species being a representative of the Osmundaceae and not, as Stur believed, of the Marattiaceae. Seeing that Lonchopteris is a designation of a purely provisional kind, it would be convenient to institute a new generic name for Triassic species having the Lonchopteris venation, which there are good reasons for regarding as Osmundaceous ferns.

Similarly Speirocarpus tenuifolius (Emmons) (= Acrostichites tenuifolius Font.), which resembles Todites Williamsoni (see p. 339) not only in habit and in the distribution of the sporangia but also in the venation, is probably an Osmundaceous species.

Osmundites.

Osmundites Dunlopi, Kidston and Gwynne-Vaughan[784], [fig. 252].

This species was found in Jurassic rocks in the Otago district of New Zealand in association with Cladophlebis denticulata[785] ([fig. 257]). The type-specimen forms part of a stem 17 mm. in diameter surrounded by a broad mass of crowded leaf-bases. The stele consists of an almost continuous xylem ring ([fig. 252]) enclosing a wide pith: the phloem and inner cortex are not preserved but the peripheral region of the stem is occupied by a sclerotic outer cortex. The mass of encasing leaf-bases resolves itself on closer inspection into zones of foliage-leaf petioles and the petioles of scale-leaves with an aborted lamina. A similar association of two forms of leaf is seen in the existing American species Osmunda Claytoniana and O. cinnamomea. The cortex and armour of leaf-bases are penetrated by numerous diarch roots. The xylem cylinder, six to seven tracheae broad, is characterised by the narrower diameter of its innermost elements and—an important point—by the fact that the detachment of a leaf-trace does not break the continuity of the xylem cylinder ([fig. 252]). Each leaf-trace is at first elliptical in section; it then becomes curved inwards and gradually assumes the horse-shoe form as in Zalesskya and in the recent species. The single endarch protoxylem becomes subdivided until in the petiole it is represented by 20 or more strands.

Fig. 252. Osmundites Dunlopi Kidst. and G.-V. Portion of xylem showing the departure of a leaf-trace. (After Kidston and Gwynne-Vaughan; × 36.)

In the continuity of the xylem cylinder this species of Osmundites shows a closer approach to Todea barbara or T. superba ([fig. 221], B) than to species of Osmunda; it differs from Zalesskya in having reached a further stage in the reduction of a solid protostele to one composed of a xylem cylinder enclosing a pith. This difference is of the same kind as that which distinguishes the stele of Lepidodendron rhodumnense from L. Harcourtii. In Lepidodendron short tracheae occasionally occur on the inner edge of the xylem cylinder, and in recent species of Todea the same kind of reduced tracheae are met with on the inner edge of the xylem[786]. In both cases the short tracheae are probably vestiges of an axial strand of conducting elements which in the course of evolution have been converted into parenchymatous cells. In Lepidodendron vasculare the mixed parenchyma and short tracheae in the centre of the stele represent an intermediate stage in xylem reduction, and the arrangement in vertical rows of the medullary parenchyma in Lepidodendron is precisely similar to that described by Kidston and Gwynne-Vaughan in Thamnopteris. In both cases the rows of superposed short cells have probably been produced by the transverse septation of cells which began by elongating as if to form conducting tubes and ended by assuming the form of vertical series of parenchymatous elements.

Fig. 253. Osmundites Kolbei Sew. (⅓ nat. size.)

In another Jurassic species, Osmundites Gibbiana[787], the xylem is of the Osmunda type and consists of about 20 strands instead of a continuous or almost continuous cylinder.

Fig. 254. Osmundites Kolbei. (Leaf-scars.)

Osmundites Kolbei Seward, [figs. 253–255].

This species was founded on a specimen obtained by Mr Kolbe from the Uitenhage series of Cape Colony[788]. The fossil flora and fauna of this series point to its correlation with the Wealden or Neocomian strata of Europe[789]. The type-specimen consists of several pieces of a stem ([fig. 253]) which reached a length of about 90 cm. On the weathered surface the remains of petiole-bases are clearly seen and on the reverse side of the smaller piece shown in the figure numerous sinuous roots are present in association with the leaf-stalks. The depression c in the larger specimen may mark the position of a branch: at a [fig. 253] (enlarged in [fig. 254], a) the vascular strand of a petiole is exposed as a broad U-shaped band and at b ([fig. 254], b) the form of the petiole-bases is clearly shown[790]. With the stem were found imperfectly preserved impressions of fronds referred to Cladophlebis denticulata, a common type of leaf which was found also in association with the slightly older New Zealand stem, Osmundites Dunlopi.

Fig. 255. Osmundites Kolbei Sew. Transverse section, from a photograph supplied by Dr Kidston and Mr Gwynne-Vaughan. (2½ nat. size.)

An examination of the internal structure of the South African stem by Dr Kidston and Mr Gwynne-Vaughan has revealed many interesting features, which will be fully described in Part IV. of their Monograph on fossil Osmundaceous stems. I am greatly indebted to these authors for allowing me to publish the following note contributed by Dr Kidston:—

“The section of Osmundites Kolbei Seward, shown in [fig. 255], presents the usual appearance of an Osmundaceous stock. The parts contained in this section are the stele, inner and outer cortex and a portion of the surrounding mantle of concrescent leaf-bases. The whole specimen has suffered much from pressure, but if restored to its original form the xylem ring must have been about 19 mm. in diameter. The number of xylem strands is about fifty-six and several of them are more or less joined as in the modern genus Todea. The tracheae are of the typical Osmundaceous type, that is to say, the pits are actual perforations and several series of them occur on each wall of the larger tracheae.

“The most interesting structural characteristic of Osmundites Kolbei is not well seen in the figure owing to the compression of the xylem ring. This consists in the occurrence of tracheae in the pith. In fact, we have here a mixed pith, composed of parenchyma and true tracheae, a condition which connects the Osmundaceae with a parenchymatous medulla with those possessing a solid xylem stele like Zalesskya and Thamnopteris and so completes the series of transitions extending from the older and solid-steled forms to the modern medullated members of the Osmundaceae.”

Osmundites skidegatensis, Penhallow.

This lower Cretaceous Canadian species, first described by Penhallow[791] and more recently by Kidston and Gwynne-Vaughan[792], is remarkable for the large size of the stem, the stele alone having a diameter of 2·4 cm. Penhallow figures a fragment of a leaf bearing a superficial resemblance to that of Osmunda Claytoniana, which may be the foliage borne by Osmundites skidegatensis. The xylem cylinder is broken by the exit of leaf-traces into 50 or more strands varying in size and shape, and it is noteworthy that the phloem is also interrupted as each leaf-trace is given off. In recent species the xylem cylinder is almost always interrupted, but the phloem retains its continuity. In the Canadian fossil an internal band of phloem occurs between the xylem and the pith, and this joins the external phloem at each leaf-gap. This internal phloem finds an interesting parallel in certain recent species[793], but in these the internal and external phloem do not meet at the foliar gaps as they do in the extinct type. In Osmunda cinnamomea the internal phloem occurs only at the regions of branching of the stem stele; in the fossil it is always present.

It is clear that Osmundites skidegatensis represents the most complex type of stem so far recognised in the Osmundaceae; it illustrates a stage in elaboration of the primitive protostele in advance of that reached by any existing species.

•••••

The primitive Osmundaceous stele was composed of solid xylem surrounded by phloem (Thamnopteris and Zalesskya); at a later stage the xylem cylinder lost its inner zone of wide and short tracheae and assumed the form seen in Osmundites Kolbei, in which the centre of the stele consists of parenchyma with some tracheae. Another type is represented by O. Dowkeri in which the pith is composed wholly of parenchyma and the xylem ring is continuous. From this type, by expansion of the xylem ring and by the formation of overlapping leaf-gaps, the form represented by Osmunda regalis was reached. Osmunda cinnamomea, with internal phloem in the regions of stelar branching, probably represents a further stage, as Kidston and Gwynne-Vaughan believe, in increasing complexity due to the introduction of phloem from without through gaps produced by the branching of the stele. In Osmundites skidegatensis the leaf-gaps became wider and the external phloem projected deeper into the stele until a continuous internal phloem zone was produced. This most elaborate type proved less successful than the simpler forms which still survive.

Osmundites Sturii.

Impressions of fertile pinnae with narrow linear segments bearing exannulate sporangia described by Raciborski from Lower Jurassic rocks in Poland as Osmunda Sturii[794] may with some hesitation be included in the list of Mesozoic Osmundaceae.

Osmundites Dowkeri.

Under this name Carruthers[795] described a petrified stem from Lower Eocene beds at Herne Bay, which in the structure of the stele agrees closely with the Jurassic species O. Gibbiana and conforms to the normal Osmundaceous type. It is possible, as Gardner and Ettingshausen[796] suggested, that the foliage of this species may be represented by some sterile Osmunda-like fragments recorded from the Middle Bagshot beds of Bovey Tracey and Bournemouth as Osmunda lignitum.

Todites.

This generic name[797] has been applied to fossil ferns exhibiting in the structure of the sporangia and in the general habit of the fertile fronds a close resemblance to the recent species Todea barbara ([fig. 221], D, p. 286).

Todites Williamsoni (Brongniart) [figs. 256], B, C, G.
1828.Pecopteris Williamsonis, Brongniart, Prodrome, p. 57; Hist. vég. foss., p. 324, Pl. CX. figs. 1 and 2.
—P. whitbiensis, Brongniart, Hist. vég. foss. p. 321, Pl. CIX. figs. 2–4.
—P. tenuis, ibid. p. 322, Pl. CX. figs. 3, 4.
1829.Pecopteris recentior, Phillips, Geol. Yorks. p. 148, Pl. VIII. fig. 15.
—P. curtata, ibid. Pl. VIII. fig. 12.
1833.Neuropteris recentior, Lindley and Hutton, Foss. Flora, Vol. I. Pl. LXVIII.
—Pecopteris dentata, ibid. Vol. III., Pl. CLXIX.
1836.Acrostichites Williamsonis, Goeppert, foss. Farn. p. 285.
1841.Neuropteris Goeppertiana, Muenster, in Goeppert, Gattungen foss. Pflanz. Lief. 5 and 6, p. 104, Pls. VIII.–X.
1856.Pecopteris Huttoniana, Zigno, Flor. foss. Oolit. Vol. I. p. 133.
1867.Acrostichites Goeppertianus, Schenk, Foss. Flor. Grenzsch. p. 44, Pl. V. fig. 5, Pl. VII. fig. 2.
1883.A. linnaeaefolius, Fontaine, Older Mesoz. Flora Virginia, p. 25, Pls. VI.–IX.
—A. rhombifolius, ibid. Pls. VIII. XI.–XIV.
1885.Todea Williamsonis, Schenk, Palaeont. Vol. XXXI. p. 168, Pl. III. fig. 3.
1889.Cladophlebis virginiensis, Fontaine, Potomac Flora, p. 70, Pl. III. figs. 3–8; Pl. IV. figs. 1, 4.

Fig. 256.

[B, C, from specimens (13491; 39234) in the British Museum (B, very slightly reduced; C, ½ nat. size); D, E, after Zeiller; F, after Renault; G, after Raciborski.]

It is hopeless to attempt to arrive at satisfactory conclusions in regard to the applicability of the name Todites Williamsoni to the numerous fronds from Jurassic and Rhaetic rocks, agreeing more or less closely with Brongniart’s type-specimen. Specimens from the Rhaetic may not be specifically identical with those from the Jurassic; the main point is that, whether actually identical or not, both sets of fossils clearly represent the same general type of Osmundaceous fern[798] and may for present purposes be included under the same designation. The above synonymy, though by no means complete[799], serves to illustrate the confusion which has existed in regard to this widely spread type of Mesozoic fern.

Todites Williamsoni may be briefly described as follows:—

Frond bipinnate; long linear pinnae (20–30 cm.) of uniform breadth arise at an acute angle, or in the lower part of a frond, almost at right angles, from a stout rachis. Closely set pinnules attached by a broad base; slightly falcate, the side towards the rachis strongly convex and the outer margin straight or concave and bulged outwards towards the base of each segment, margin usually entire, or it may be slightly lobed. Fertile pinnules similar to the sterile; sporangia of the Osmundaceous type and often scattered over the whole lower surface of the lamina ([fig. 256], B, B′, G). Venation of the Cladophlebis type (cf. [fig. 256], A).

It is not always easy to distinguish Todites Williamsoni from Cladophlebis denticulata, another common Jurassic fern, but in the latter the pinnules are usually longer and relatively narrower and the rachis is more slender (cf. [fig. 256], B and 257). Schenk[800] and Raciborski[801] have shown that the sporangia of Todites conform in the absence of a true annulus to those of Todea ([fig. 256], G) and Osmunda. Nathorst[802] has recently figured a group of spores of Todites Williamsoni in illustration of the use of the treatment of carbonised impressions with nitric acid and potassium chlorate. This species, though widely distributed in Jurassic rocks, is hardly distinguishable from the German Rhaetic fronds figured by Schenk from Bayreuth as Acrostichites Goeppertianus[803], or from other fossils referred to an unnecessarily large number of species by Fontaine[804] from Upper Triassic rocks of Virginia[805].

It would seem from the paucity of later records of Osmundaceae that the family reached its zenith in the Jurassic era. When we pass to the later Tertiary and more recent deposits evidence is afforded in regard to the geographical range of Osmunda regalis. It has been shown to occur in the Pliocene forest-bed of Norfolk[806] as well as in Palaeolithic and Neolithic deposits[807].

Fig. 257. Cladophlebis denticulata. (From a specimen in the British Museum from the Inferior Oolite rocks of Yorkshire. Slightly reduced.)

A fertile frond from the Molteno (Rhaetic) beds of South Africa referred to Cladophlebis (Todites) Roesserti (Presl)[808] represents in all probability an Osmundaceous fern closely allied to Todites Williamsoni. The same species is described by Zeiller[809] from Rhaetic rocks of Tonkin and very similar types are figured by Leuthardt[810] from Upper Triassic rocks of Basel as Pecopteris Rutimeyeri Heer, and by Fontaine[811] from rocks of the same age in Virginia.

Cladophlebis.

The generic name Cladophlebis was instituted by Brongniart for Mesozoic fern fronds characterised by ultimate segments of linear or more or less falcate form attached to the pinnae by the whole of the base, as in the Palaeozoic genus Pecopteris, possessing a midrib strongly marked at the base and dividing towards the distal end of the lamina into finer branches and giving off secondary forked and arched veins at an acute angle. The term is generally restricted to Mesozoic fern fronds which, on account of the absence or imperfection of fertile pinnae, cannot be safely assigned to a particular family. In the case of the species described below, the evidence in regard to systematic position, though not conclusive, is sufficiently strong to justify its inclusion in the Osmundaceae.

Cladophlebis denticulata Brongniart. Figs. [256], A; [257], [258].
1828.Pecopteris denticulata[812], Brongniart, Prodrome, p. 57; Hist. vég. foss. p. 301, Pl. XCVIII. figs. 1, 2.
—P. Phillipsii, Brongniart, Hist. p. 304, Pl. CIX. fig. 1.

This species is often confused[813] with Todites Williamsoni. The name Pecopteris whitbiensis has been used by different writers for Jurassic fronds which are undoubtedly specifically distinct: specimens so named by Brongniart should be referred to Todites Williamsoni, while P. whitbiensis of Lindley and Hutton[814] is Brongniart’s Cladophlebis denticulata. It is impossible to determine with accuracy the numerous examples described as Pecopteris whitbiensis, Asplenium whitbiense, Cladophlebis Albertsii (a Wealden species[815]), Asplenium, or Cladophlebis, nebbense[816], etc., from Jurassic and Rhaetic strata. The Cladophlebis denticulata form of frond is one of the commonest in recent ferns; it is represented by such species as Onoclea Struthopteris, Pteris arguta, Sadleria sp., Gleichenia dubia, Alsophila lunulata, Cyathea dealbata, and species of Polypodium. It is, therefore, not surprising to find records of this Mesozoic species from many localities and horizons. All that we can do is to point out what appear to be the most probable cases of identity among the numerous examples of fronds of this type from Mesozoic rocks, particularly Rhaetic and Jurassic, in different parts of the world. The name Cladophlebis denticulata may be employed in a comprehensive sense for fronds showing the following characters:—

Leaf large, bipinnate, with long spreading pinnae borne on a comparatively slender rachis. Pinnules, in nearly all cases, sterile, reaching a length of 3–4cm., acutely pointed, finely denticulate or entire, attached by the whole of the base ([fig. 257]). In the apical region the pinnules become shorter and broader. Venation of the Cladophlebis type ([fig. 256], A). Fertile pinnules rather straighter than the sterile, characterised by linear sori parallel to the lateral veins ([fig. 258]).

In endeavouring to distinguish specifically between fronds showing a general agreement in habit with C. denticulata, special attention should be paid to venation characters, the shape of the pinnules, the relation of the two edges of the lamina to one another, and to the amount of curvature of the whole pinnule. Unless the material is abundant, it is often impossible to distinguish between characters of specific value and others which are the expression of differences in age or of position on a large frond, to say nothing of the well-known variability which is amply illustrated by recent ferns. It is remarkable that very few specimens are known which throw any light on the nature of the fertile pinnae. Fig. 258 represents an impression from the Inferior Oolite rocks of the Yorkshire coast in which the exposed upper surface of the pinnules shows a series of parallel ridges following the course of the lateral veins and no doubt formed by oblong sori on the lower surface. There can be little doubt that the specimen figured by Lindley and Hutton and by others as Pecopteris undans[817] is, as Nathorst suggests, a portion of a fertile frond of C. denticulata. A fertile specimen of a frond resembling in habit C. denticulata, which Fontaine has described from the Jurassic rocks of Oregon as Danaeopsis Storrsii[818], exhibits, as that author points out, a superficial resemblance to the specimen named by Lindley and Hutton Pecopteris undans. There is, however, no adequate reason for referring the American fragment to the Marattiaceae. In the absence of sporangia we cannot speak confidently as to the systematic position of this common type; but there are fairly good grounds for the assertion that some at least of the fronds described under this name are those of Osmundaceae. The English specimen shown in [fig. 258] is very similar to some Indian fossils figured by Feistmantel as Asplenites macrocarpus[819], which are probably identical with Pecopteris australis Morris[820], a fern that is indistinguishable from Cladophlebis denticulata. Renault[821] figured a fertile specimen of the Australian fossil as Todea australis, which agrees very closely with that shown in [fig. 258], and the sporangia figured by the French author are of the Osmundaceous type. Another example of a fertile specimen is afforded by a Rhaetic fern from Franconia, Asplenites ottonis, which is probably identical with Alethopteris Roesserti Presl [= Cladophlebis (Todites) Roesserti], a plant closely resembling Cladophlebis denticulata. Another argument in favour of including C. denticulata in the Osmundaceae is supplied by the association of pinnae of this type with the petrified stem of Osmundites Dunlopi recorded by Kidston and Gwynne-Vaughan.

Fig. 258. Fertile pinnae of Cladophlebis denticulata. (From a Yorkshire specimen in the Sedgwick Museum, Cambridge.)