Chapter XXI: Fossil Ferns (2)

The specimens from the Culm rocks of Moravia on which Stur founded the species _Thyrsopteris schistorum_[896] are too imperfectly preserved to warrant the use of this generic name. Goeppert[897] in 1836 instituted the genera _Cyatheites_, _Hemitelites_, and _Balantites_ for species of Carboniferous ferns believed to be closely allied to recent Cyatheaceae, but a fuller knowledge of these types has clearly demonstrated that in all cases the reference to this family had no justification.

The Upper Carboniferous species _Dicksonites Pluckeneti_, of which Sterzel[898] described fertile specimens in 1886 as possessing circular sori, has since been shown by Grand’Eury[899] to be a Pteridosperm bearing small seeds. In _Sphenopteris_ (_Discopteris_) _cristata_ (Brongn.) Zeiller[900] has described sori very like those of _Cyathea_ and _Alsophila_, but differing in the exannulate sporangia: this species, like so many of the Palaeozoic ferns, is probably more akin to the Marattiaceae than to the Cyatheaceae.

We have as yet no satisfactory evidence of the existence of the Cyatheaceae in Palaeozoic floras. It is not until we reach the Jurassic period that trustworthy data are obtained. Raciborski[901] has identified as Cyatheaceous fertile Jurassic fronds from Poland, but his figures are inconclusive. In _Alsophila polonica_ it is not clear whether the annulus is vertical or oblique, and in another supposed member of the family, _Gonatosorus Nathorsti_, in which the indusium is described as bivalvate, there is no proof of affinity to Cyatheaceae.

In attempting to decipher the past history of the Cyatheaceae it is important to remember the close resemblance between the fertile segments of some species of _Davallia_ (Polypodiaceae) and those of _Dicksonia_ (fig. 229, C, D, p. 294). Unless the sporangia are well enough preserved to show the position of the annulus, it is frequently impossible to feel much confidence in the value of the grosser features, such as the reduced lamina of the fertile segments and the form of the sori. It is, however, probable that the widely-spread Jurassic species _Coniopteris hymenophylloides_ is correctly referred to the Cyatheaceae, but even in the case of this species the evidence of external form needs confirmation by an examination of individual sporangia.

_Coniopteris._

This genus was instituted by Brongniart[902] for fossil fronds characterised by pinnules more or less intermediate between the _Pecopteris_ and _Sphenopteris_ type and agreeing in the form of the sori with the leaves of recent species of _Dicksonia_. It should be noted that Stur included in this genus a species, _Coniopteris lunzensis_[903] from the Upper Trias of Lunz, which he regarded as a Marattiaceous fern.

_Coniopteris hymenophylloides_, Brongn. Figs. 271, 272, 275, B.

1828. _Sphenopteris hymenophylloides_, Brongniart, Hist. vég. foss.
p. 189, Pl. LVI. fig. 4.

1829. _S. stipata_, Phillips, Geol. York. p. 147, Pl. X. fig. 8.

1835. _Tympanophora simplex_, Lindley and Hutton, Foss. Flor. Pl.
CLXX. A.

— _T. racemosa_, _ibid._ Pl. CLXX. B.

— _Sphenopteris arguta_, _ibid._ Pl. CLXVIII.

1836. _Hymenophyllites Phillipsi_, Goeppert, Foss. Farn. p. 256.

1849. _Coniopteris hymenophylloides_, Brongniart, Tableau, p. 105.

— _Coniopteris Murrayana_, _ibid._

1851. _Sphenopteris nephrocarpa_, Bunbury, Quart. Journ. Geol. Soc.
Vol. VII. p. 129, Pl. XII. fig. 1.

1876. _Thyrsopteris Murrayana_, Heer, Flor. Foss. Arct. Vol. IV. (2)
p. 30, Pls. I. II. VIII.

The above list represents a small selection of the names applied to Jurassic ferns from different localities which there are good grounds for regarding as referable to a single type[904].

Frond tripinnate; pinnae linear acuminate, attached to the rachis
at a wide angle; the pinnules vary considerably in size and
shape; in some the lamina is divided into a few broad and rounded
lobes (fig. 275, B) while in others the leaflets are dissected
into narrow linear segments. The sori are borne at the ends of
veins; the fertile pinnules have a much reduced lamina and, in
extreme cases, bear a close resemblance to those of _Thyrsopteris
elegans_ (fig. 229, A, p. 294). The sori are partially enclosed in
a cup-like indusium and the sporangia appear to have an oblique
annulus.

Venation and habit of frond of the _Sphenopteris_ type.

The pinna shown in fig. 271 is the type-specimen of _Sphenopteris arguta_ Lind. and Hutt. from the Yorkshire Inferior Oolite and is indistinguishable from the English examples on which Brongniart founded his species _S. hymenophylloides_. Fig. 272 shows a specimen from the York Museum illustrating the difference between the sterile and fertile pinnae. The resemblance of some fertile pinnae of _Coniopteris hymenophylloides_ to those of _Thyrsopteris elegans_ has led to a frequent use, without any solid justification, of the generic name of the Juan Fernandez fern for Jurassic and Wealden plants. It is not impossible that some of the fossils described by Heer from Jurassic rocks of Siberia[905] as species of _Thyrsopteris_ are Cyatheaceous ferns, but it is impossible to say with certainty that they are generically identical with the recent species. In his monograph of the Potomac flora of Virginia[906] and Maryland, Fontaine has described as species of _Thyrsopteris_ several specimens of fronds which afford no evidence as to the nature of the sori or sporangia. Some of the fronds referred by this author to _Thyrsopteris rarinervis_[907], which I examined in the Washington Museum, are in all probability examples of _Onychiopsis_, a genus included in the Polypodiaceae. The fragments described by Lester Ward[908] as species of _Thyrsopteris_ from the Lower Cretaceous of the Black Hills of North America afford no satisfactory evidence of relationship to the recent type. Similarly Velenovský has described a Lower Cretaceous _Onychiopsis_ from Bohemia[909] as a species of _Thyrsopteris_, although the fertile segments bear little or no resemblance to those of the Cyatheaceous genus. Some fertile portions of fronds described by Heer[910] as _Asplenium Johnstrupi_ and afterwards as _Dicksonia Johnstrupi_[911] from the Cretaceous beds (Kome series) of Greenland are very similar to _Coniopteris hymenophylloides_.

]

_Coniopteris quinqueloba_ (Phillips). Fig. 273.

This species, originally described by Phillips[912] as _Sphenopteris quinqueloba_, is very similar in habit to _C. hymenophylloides_, differing chiefly in the smaller size of the leaf and in the narrower ultimate segments. The specimen shown in fig. 273, B, illustrates the form of the sorus and sporangia.

_Coniopteris arguta_ (Lind. and Hutt.[913]). Figs. 274, 275, A.

The sterile pinnae of this species bear pinnules of a type met with in various species of ferns from different horizons; the smaller ones are entire and slightly falcate, while on the lower part of a frond the ultimate segments are longer and have a crenulate margin. The fertile pinnae bear pinnules reduced to a midrib with a narrow border, and terminating in a cup-like indusium (fig. 275, A). In habit the sterile leaf (fig. 274) of this species is similar to the Jurassic Schizaeaceous fern _Klukia exilis_.

_Protopteris._

Presl[914] instituted this genus for a Lower Cretaceous tree-fern from Bohemia originally figured as _Lepidodendron punctatum_[915] and assigned to a Palaeozoic horizon; it was afterwards named by Corda[916] _Protopteris Sternbergii_ and referred by Brongniart[917] to _Sigillaria_. The genus _Protopteris_ stands for fossil fern-stems with the habit and, in the main, the structural features of recent tree-ferns. Persistent leaf-bases and sinuous adventitious roots cover the surface of the stems: the vascular system is of the dictyostelic type characteristic of _Cyathea_ (fig. 240, p. 313) and _Alsophila_. It is by the pattern formed by the vascular tissue on the exposed surface of the leaf-bases that _Protopteris_ is most readily recognised: the leaf-trace has a horse-shoe form with the ends curled inwards and the sides more or less indented (fig. 277). The generic name _Caulopteris_ is used by some authors in preference to Presl’s genus; but _Protopteris_ is more conveniently restricted to Mesozoic Cyatheaceous stems and _Caulopteris_ to Palaeozoic stems, with the internal structure of _Psaronius_ (see Chap. XXIII.). Stenzel applies _Caulopteris_ to Mesozoic stems in which the leaf-trace consists of several separate strands and not of a continuous band.

A. _Coniopteris arguta._ (Fertile pinnae; nat. size.)
B. _C. hymenophylloides._

A, from the Inferior Oolite of Yorkshire (British Museum); B, from
Jurassic rocks in Turkestan. ]

Lower Cretaceous casts of tree-fern stems in the Prague Museum have been described under the names _Alsophilina_ and _Oncopteris_; the figures of the latter (fig. 276) given by Feistmantel[918] and by Velenovský[919] show the petiole-bases arranged in vertical rows and characterised by leaf-traces consisting of two separate strands in the form of two =V=s lying on their sides.

Tree-fern stems described under various generic names are not infrequently found in European Lower Cretaceous rocks: their comparative abundance affords an example of striking changes in geographical distribution since the latter part of the Mesozoic epoch. The Cyatheaceae no longer exist in Europe and the arborescent species of the genus have retreated to more southern regions.

_Protopteris punctata_ (Sternb.). Fig. 277.

The earliest information in regard to the anatomy of this widely spread Lower Cretaceous fern we owe to Corda, who showed that the species agrees in essentials with existing tree-ferns. The English example described by Carruthers[920] from Upper Greensand beds in Dorsetshire (now in the British Museum) shows only the external features. The sandstone cast (14 cm. in diameter), of which a portion is seen in fig. 277, was described by Heer from Disco Island (Greenland) as a Carboniferous species[921], but afterwards correctly assigned to the Cenomanian series[922] This species is recorded also from the Lower Cretaceous of Bohemia by Frič and Bayer[923] Among examples of petrified stems exhibiting a general agreement with _Protopteris punctata_ are those described by Stenzel[924] from Turonian rocks in Germany. In one of these, _Rhizodendron oppoliense_ Göpp., attention is drawn to branches given off from the stem stele which have a solenostelic structure in contrast to the dictyostele of the stem; also to the minute structure of the tracheae which appear to have their ends perforated, a feature shown by Gwynne-Vaughan[925] to be characteristic of the xylem elements of many ferns.

A. _Laccopteris polypodioides_, Brongn. [From a specimen (39275) in
the British Museum; slightly reduced.]
B. _L. Muensteri._
C. _Dicksonia_ (petiole stele).
D. _Onychiopsis Mantelli_ (fertile segments).
E. _Hausmannia Sewardi_ Richt.
F. _H. Kohlmanni_ Bicht.
G, H. _Protopteris Witteana_, Schenk. (x, xylem; R, roots.)

(B, after Schenk; E, F, after Richter.)]

_Protopteris Witteana_ Schenk[926] (fig. 278, G, H), a Wealden species recorded from Germany and England, represents a closely allied or possibly an identical type. The section of the stem (fig. H) shows the narrow vascular bands, x, of a dictyostele similar to that of recent Cyatheaceous tree-ferns and a form of meristele (fig. G, x) resembling that of _P. punctata_. Adventitious roots are seen in section at R (figs. G and H).

=Polypodiaceae.=

Sections of petrified sporangia from the English Coal-Measures (_Pteridotheca_ sp.) occasionally exhibit a striking resemblance to those of recent Polypodiaceae[927], but in the absence of material in which it is possible to recognise the true orientation of the sporangia, the exact position of the annulus is almost impossible to determine. We have as yet no satisfactory evidence of the existence of true Polypodiaceae in the Palaeozoic era. It is noteworthy that apart from the absence of ferns which can reasonably be included in this family, the anatomical features of the Botryopterideae (Coenopterideae) and of the Cycadofilices or Pteridosperms do not foreshadow those of Polypodiaceous ferns. On the other hand, as we have already noticed, anatomical characters of such families as the Gleicheniaceae, Hymenophyllaceae, and Schizaeaceae are met with in certain generalised Palaeozoic types. These facts are perhaps of some importance as supplying collateral evidence in favour of the relatively more recent origin of the dominant family of ferns in modern floras.

(A, after Kidston.)]

The use of the generic name _Adiantites_ for fern-like fronds of Lower Carboniferous age characterised by cuneate pinnules like those of species of _Adiantum_, suggests an affinity which is in all probability non-existent. It has been pointed out that this generic name was applied in the first instance to the leaves of the Jurassic plant _Ginkgo digitata_[928] and should, therefore, be discarded. Schimper[929] used the designation _Adiantides_, and Ettingshausen[930], more rashly than wisely, preferred _Adiantum_. The specimens described by Kidston[931] as _Adiantides antiquus_ (Ett.) (fig. 279, A) from the Carboniferous limestone of Flintshire are portions of tripinnate fronds bearing cuneate segments with numerous forked veins radiating from the contracted base of the lamina. It is not improbable, in view of Dr White’s[932] discovery of seeds on a very similar plant from the Pottsville beds of North America, that this characteristic Lower Carboniferous genus is a Pteridosperm.

From Jurassic rocks in various parts of the world numerous fossils have been described under the generic names _Aspidium_, _Asplenium_, _Davallia_, _Polypodium_, and _Pteris_. In the great majority of cases such records leave much to be desired from the point of view of students who appreciate the dangers of relying on external similarity between vegetative organs, and on resemblances founded on obscure impressions of sori. The generic term _Woodwardites_[933], which suggests affinity with the recent genus _Woodwardia_, has been used for Rhaetic plants belonging to the Dipteridinae.

A plant described as _Adiantides Lindsayoides_ from Jurassic rocks of Victoria[934], characterised by marginal sori which appear to be protected by the folded-over edge of the leaflets, and by the resemblance of the pinnules to those of recent species of _Lindsaya_, may be a true Polypodiaceous fern; but in this case, as in many similar instances, nothing is known of the structure of the sporangia. Some sterile pinnae described by Yabe from Jurassic rocks of Korea as _Adiantites Sewardi_[935] may perhaps be identical with the Australian species.

In such a species as _Polypodium oregonense_ Font., from Jurassic rocks of Oregon, the generic name is chosen because the “fructification seems near enough to that of _Polypodium_ to justify the placing of the plant in that genus[936].” But the fact that no sporangia have been found is a fatal objection to this identification.

_Onychiopsis._

This generic name was instituted by Yokoyama[937] for a Japanese Wealden species, previously described by Geyler[938] as _Thyrsopteris elongata_, on the ground that, in addition to a similarity in habit of the sterile fronds, the fertile pinnae present a close agreement to those of the recent genus _Onychium_.

_Onychiopsis Mantelli_[939] (Brongn.). Figs. 278, D; 280, A and B.

The Japanese species _Onychiopsis elongata_ may perhaps be identical with this common Wealden fern which, as Fontaine points out, should be called _O. psilotoides_ if the rule of priority is to be observed irrespective of long usage.

1824. _Hymenopteris psilotoides_, Stokes and Webb, Trans. Geol. Soc.
[ii.], Vol. I. p. 423, Pl. XLVI. fig. 7.

1828. _Sphenopteris Mantelli_, Brongniart, Hist. vég. foss. p. 170,
Pl. XLV. figs. 3–7.

1890. _Onychiopsis Mantelli_, Nathorst, Denksch. Wien Akad. Vol.
LVII. p. 5.

_Onychiopsis Mantelli_ may be defined as follows:—

Frond bipinnate, ovate lanceolate, rachis winged; pinnae
approximate, given off at an acute angle; pinnules narrow,
acuminate, with a single vein; the larger segments serrate and
gradually passing into pinnae with narrow ultimate segments.
Fertile segments sessile or shortly stalked, linear ovate,
sometimes terminating in a short awn-like prolongation.

The fertile segments (fig. 278, D) bear so close a resemblance to those of species of _Onychium_ that it would seem justifiable to regard the plant as a member of the Polypodiaceae. This fern is one of the most characteristic members of the Wealden floras; it occurs in abundance in the English Wealden, in Portugal, Germany, Belgium, Japan, Bohemia, South Africa, and elsewhere. A piece of rhizome figured from the English Wealden[940] is very similar to the creeping rhizomes of recent species of Polypodiaceae. The English Wealden specimens shown in fig. 280, A and B, illustrate the difference in form presented by leaves of this species; the smaller pinnae reproduced in fig. A are more characteristic of the species than are those of the slightly enlarged example represented in fig. 280, B.

Among British Tertiary species referred to Polypodiaceae, it is interesting to find what may well be an authentic record of a fern closely allied to the recent tropical species _Acrostichum_ (_Chrysodium_) _aureum_. This Eocene species from Bournemouth is described as _Chrysodium lanzaeanum_[941]. The frond is simply pinnate and apparently coriaceous in texture, with lanceolate or oblong lanceolate pinnules (fig. 261, A, A′, p. 350), differing from those of _Acrostichum aureum_ in being sessile. A prominent midrib gives off numerous anastomosing veins. No fertile pinnules have been found.

Specimens described by Forbes from the Eocene beds of the Island of Mull as _Onoclea hebraidica_[942] bear a strong likeness to the North American and Japanese recent species _Onoclea sensibilis_. Fertile specimens referred to the latter species are recorded by Knowlton[943] from Tertiary beds of Montana.

A species described by Saporta[944] from the Eocene of Sézanne as _Adiantum apalophyllum_ is recorded by Gardner and Ettingshausen from Bournemouth; an identification which is based on somewhat meagre evidence.

The following remarks by Gardner and Ettingshausen are worthy of repetition as calling attention to circumstances often overlooked in analyses of fossil floras. They speak of ferns as relatively rare in British Eocene rocks and add,—“the floras consist principally of deciduous dicotyledonous leaves, which ... fell into the water and were tranquilly silted over. Ferns, on the other hand, would require some violence to remove them from the place of their growth, and their preservation would consequently be exceptional, and they would be mutilated and fragmentary. This may account for their rarity. Few as the British ferns are in the number of species, they nevertheless form the largest and most important series of Eocene ferns, even of Tertiary ferns, yet described from one group of beds[945].”

=Dipteridinae.=

_Dictyophyllum._

This genus was founded by Lindley and Hutton for a pinnatifid leaf from the Jurassic rocks of Yorkshire which they regarded as probably dicotyledonous and named _D. rugosum_[946]. Several ferns of this genus have since been found with well-preserved sori which demonstrate a close similarity to the recent fern _Dipteris._ _Dictyophyllum_ may be defined as follows:—

Fronds large and palmate, characterised by the equal dichotomy of the main rachis into two arms which curve outwards and then bend inwards (fig. 281); from the surface of each arm are given off numerous spreading pinnae with a lamina more or less deeply dissected into lobes varying in breadth and in the form of the apex. Each lobe has a median vein, from which branches are given off approximately at right angles and then subdivide into a reticulum, in the meshes of which the veinlets end blindly (fig. 282, A and E). Sori composed of annulate sporangia are crowded on the lower surface of the lamina. In habit and in sporangial characters the genus closely resembles _Dipteris_, and in the branching of the frond suggests comparison with _Matonia_. The rhizome (_Rhizomopteris_) is creeping and dichotomously branched, bearing leaf-scars with a horse-shoe form of vascular strand.

_Dictyophyllum_ is represented by several types to which various specific names have been assigned, the distinguishing features being the form of the pinna lobes, the degree of concrescence between the basal portions of the pinnae, and similar features which in some cases can only be safely used as criteria when large specimens are available for comparison.

_Dictyophyllum exile_ (Brauns). Figs. 281, 282, D, E.

1862. _Camptopteris exilis_, Brauns, Palaeontograph. IX. p. 54.

1867. _Dictyophyllum acutilobum_, Schenk, Foss. Flor. Grenz. p. 77,
Pls. XIX. XX.

1878. _D. exile_, Nathorst, Flora vid Bjuf, I. p. 39, Pl. V. fig. 7.

—— _D. acutilobum_, _ibid._ Pl. XI. fig. 1.

The restoration, after Nathorst[947], shown in fig. 281 illustrates the habit of this striking fern, examples of which or of closely allied species are recorded from Rhaetic rocks of Germany, Scania, Persia, Bornholm, Tonkin, China, and elsewhere[948]. The petiole, reaching a length of 60 cm., forks at the apex into two equal arms leaving between them an oval space and occasionally crossing one another. The axes of these branches are twisted so that the pinnae, which may be as many as 24 on each arm, and arise from the inner side, by torsion of the axes assume an external position. An interesting analogy as regards the twisted rachis of _Dictyophyllum exile_ and _Camptopteris_ is afforded by the leaves of the Cycads, _Macrozamia Fawcettiae_ and _M. corallipes_, which are also characterised by the torsion of the rachis. The habit, justly compared by Nathorst with that of _Matonia pectinata_, affords another illustration of the common occurrence in older ferns of a dichotomous system of branching. The pinnae, characterised by circinate vernation, reach a length of 60 cm. and are divided into linear lobes inclined obliquely or at right angles to the pinna axis. The whole of the under surface of the lamina may be covered with sporangia, 4–7 sporangia in each sorus; the annulus is incomplete and approximately vertical (fig. 282, D). The rhizome is probably represented by the dichotomously branched axis described by Nathorst from Scania as _Rhizomopteris major_; the leaf-scars show a horse-shoe leaf-trace.

A. _Dictyophyllum Nilssoni._
B. _Rhizomopteris Schenki._
C. _Camptopteris spiralis._
D, E. _Dictyophyllum exile._

(After Nathorst; A, B, C, E, ⅔ nat. size.)]

_Dictyophyllum Nathorsti_ Zeiller[949].

This type, represented by a splendid series of specimens from the Rhaetic beds of Tonkin, agrees very closely with _D. exile_. It differs, however, in the basal parts of the pinnae which are concrescent for a length of 5 to 8 cm. instead of free as in _D. exile_; and, to a slight degree, in the form of the ultimate segments. In habit and in soral characters the two species are practically identical. Each sorus contains 5 to 8 sporangia, which are rather larger than those of _Dipteris_.

_Dictyophyllum rugosum_, Lind. and Hutt. Fig. 283.

1828. _Phlebopteris Phillipsii_, Brongniart, Hist. vég. foss. p. 377,
Pl. CXXXII. fig. 3; Pl. CXXXIII. fig. 1.

1829. _Phyllites nervulosis_, Phillips, Geol. Yorks. p. 148, Pl.
VIII. fig. 9.

1834. _Dictyophyllum rugosum_, Lindley and Hutton, Foss. Flor. II.
Pl. CIV.

1836. _Polypodites heracleifolius_, Goeppert, Foss. Farn. p. 344.

1849. _Camptopteris Phillipsii_, Brongniart, Tableau, p. 105.

1880. _Clathropteris whitbyensis_, Nathorst, Berättelse, p. 83.

This species, which is characteristic of Jurassic rocks, is less completely known than the two types described above, but in the form and venation of the pinnae there is little difference between the Rhaetic and Jurassic plants. The leaves of the Jurassic species appear to have been smaller and more like those of _Dipteris conjugata_ (fig. 231); there are no indications of the existence of the two curved arms at the summit of the petiole which form so striking a feature in _D. exile_ and _D. Nathorsti_. No sporangia have been found on English specimens, but it is safe to assume their agreement with those of other species. A more complete list of records of _D. rugosum_ is given in the first volume of the British Museum Catalogue of Jurassic plants[950].

Nathorst[951] has recently drawn attention to certain differences between _Dictyophyllum_ and _Dipteris_. The pinnate division of the pinnae is not represented in the fronds of the recent species, but this method of lobing, which is a marked characteristic of _Dictyophyllum_, is less prominent in _Clathropteris_; and in _Camptopteris lunzensis_ Stur[952], an Austrian Upper Triassic species, the pinnae are entire. In _Dictyophyllum_ the sori cover the whole lower surface of the leaf; in _Dipteris_ they are more widely separated and the sporangia have a diameter of 0·02 mm., but in _Dictyophyllum_ the diameter is 0·4–0·6 mm. Moreover in _Dictyophyllum_ the sori contain 5 to 8 sporangia, whereas in _Dipteris_ they are much more numerous. Despite these differences it is clear, as Nathorst says, that _Dictyophyllum_, _Clathropteris_, and _Camptopteris_ are existing types very closely allied to _Dipteris_. It is a matter of secondary importance whether we include all in the Dipteridinae or follow Nathorst’s suggestion and refer the fossil genera to the separate family Camptopteridinae.

_Thaumatopteris._

This genus, founded by Goeppert[953] for a Rhaetic plant from Bayreuth, is by some authors[954] regarded as identical with _Dictyophyllum_, but it has recently been resuscitated by Nathorst[955] for specimens which he names _T. Schenki_, formerly included by Schenk in his species _T. Brauniana_[956]. It bears a close resemblance, in the long linear pinnules with an entire or crenulate margin, to _Dictyophyllum Fuchsi_ described by Zeiller[957] from Tonkin, and it would seem hardly necessary to adopt a distinctive generic designation. The sporangia have a vertical or slightly oblique annulus and the rhizome is similar to that of _Dictyophyllum exile_. The habit of the genus is shown in fig. 284, which represents one of the German Rhaetic species.

_Clathropteris._

_Clathropteris meniscoides_, Brongn. Fig. 285.

_Clathropteris_, founded by Brongniart[958] for Rhaetic specimens from Scania, agrees very closely with some species of _Dictyophyllum_, but in view of the more rectangular form of the venation-meshes it is convenient to retain both names. The type-species was originally named _Filicites meniscoides_[959] and afterwards transferred to _Clathropteris_. An examination of Brongniart’s specimens has convinced Nathorst of the specific identity of _C. meniscoides_ and _C. platyphylla_. The Tonkin leaves described by Zeiller[960] under the latter name should, therefore, be included in _C. meniscoides_, which may be thus defined:

The petiolate frond is characterised by an equal dichotomy of
the rachis, as in _Dictyophyllum_; each branch bore 5–15 pinnae,
disposed _en éventail_, reaching a length of 20–30 cm. and fused
basally as in _D. Nathorsti_ Zeill. Pinnae linear lanceolate,
slightly contracted at the lower end and gradually tapered
distally. The lamina, 3–14 cm. broad, is characterised by obtusely
pointed marginal lobes. From the midrib of each pinna lateral
veins are given off at a wide angle, and adjacent veins are
connected by a series of branches which divide the lamina into a
regular reticulum of rectangular and polygonal meshes (fig. 285).
The sori are abundant and contain 5–12 sporangia like those of
_Dictyophyllum_.

]

What is probably the rhizome of this species has been described by Nathorst (_Rhizomopteris cruciata_); it is similar to that of _Dictyophyllum_, but the leaf-scars are more widely separated. This species occurs in Upper Triassic, Rhaetic or Lower Jurassic rocks of Scania, France, Germany, Switzerland, Bornholm, North America, China, Tonkin, and Persia and is represented by fragments in the Rhaetic beds of Bristol[961].

_Clathropteris egyptiaca_ Sew.[962] Fig. 286.

The specimen on which this species was founded was discovered in the Nubian Sandstone east of Edfu; the age of the beds is uncertain, but the presence of _Clathropteris_ suggests a Lower Jurassic or Rhaetic horizon[963]. Seven strong ribs radiate through the lamina from the summit of the petiole; at _a_ and _b_ small pieces of the projecting ribs are shown in the grooves. From the main veins slender branches are given off at right angles and, as seen in the enlarged drawing, these again subdivide into a delicate reticulum with free-ending veinlets.

_Camptopteris._

_Camptopteris spiralis_, Nath. Figs. 282, C; 287.

Nathorst proposed this generic name for Rhaetic fronds[964] resembling those of _Clathropteris_ and _Dictyophyllum_, but differing in the form of the pinnae and in habit. The habit of the type-species, _C. spiralis_, is shown in fig. 287. An examination of the specimens in the Stockholm Museum convinced me of the correctness of Nathorst’s restoration[965]. Each of the forked arms of the rachis bore as many as 150–160 long and narrow pinnae characterised by an anastomosing venation (fig. 282, C) and by a spiral disposition due to the torsion of the axes. The sporangia agree in essentials with those of _Dictyophyllum_.

_Hausmannia._

A critical and exhaustive account of this genus has been given by Prof. Von Richter[966] based on an examination of specimens found in the Lower Cretaceous rocks of Quedlinburg in Germany. The name was proposed by Dunker[967] for leaves from the Wealden of Germany characterised by a deeply dissected dichotomously branched lamina. Andrae subsequently instituted the genus _Protorhipis_[968] for suborbicular leaves with dichotomously branched ribs from the Lias of Steierdorf. A similar but smaller type of leaf was afterwards described by Zigno[969] from Jurassic beds of Italy as _P. asarifolius_, and Nathorst[970] figured a closely allied form from Rhaetic rocks of Sweden. While some authors regarded _Hausmannia_ and _Protorhipis_ as ferns, others compared them with the leaves of _Baiera_ (Ginkgoales); Saporta suggested a dicotyledonous affinity for leaves of the _Protorhipis_ type. The true nature of the fossils was recognised by Zeiller[971], who called attention to the very close resemblance in habit and in soral characters to the recent genus _Dipteris_. A comparison of the different species of _Dipteris_, including young leaves (fig. 231, p. 297), with those of the fossil species reveals a very striking agreement[972]. There can be no doubt, as Richter points out, that the names _Hausmannia_ and _Protorhipis_ stand for one generic type.

_Hausmannia_ may be defined as follows:

Rhizome creeping, slender, dichotomously branched; leaf-stalks
slender (2–25 cm. long), bearing a leathery lamina (1–12 cm. long
and broad), wedge-shaped below, occasionally cordate or reniform,
entire or more or less deeply lobed into broad linear segments.
The leaf is characterised by dichotomously branched main ribs
which arise from the summit of the rachis as two divergent arms
and radiate in a palmate manner, with repeated forking, through
the lamina. Lateral veins are given off at a wide angle, and, by
subdivision, form a fairly regular network similar to that in
_Dictyophyllum_, _Clathropteris_, and _Dipteris_.

_Hausmannia dichotoma_, Dunker[973]. Fig. 288, A, B.

This Wealden species, represented in the North German flora and in beds of approximately the same age at Quedlinburg, has been discovered by Dr Marcus Gunn in Upper Jurassic rocks on the north-east coast of Scotland. The lamina (12 cm. or more in length) is divided into five to seven linear segments and bears a close superficial resemblance to leaves of _Baiera_ and to recent species of _Schizaea_ (fig. 222, p. 287). Each segment contains one or two main ribs (fig. 288, A). A similar form is described by Bartholin[974] and by Moeller[975] as _H. Forchammeri_ from Jurassic rocks of Bornholm.

_Hausmannia Kohlmanni_, Richt. Fig. 278, F.

In this species, instituted by Richter from material obtained from the Lower Cretaceous beds of Strohberg[976], the comparatively slender rhizome bears fronds with petioles reaching a length in extreme cases of 25 cm. but usually of about 10 cm. The lamina (1–7 cm. long and 1–10 cm. broad) is described as leathery, obcordate, and divided into two symmetrical halves by a median sinus which, though occasionally extending more than half-way through the lamina, is usually shallow. The venation consists of two main branches which diverge from the summit of the petiole (fig. 278, F) and subdivide into dichotomously branched ribs; finer veins (not shown in the drawing) are given off from these at right angles and form more or less rectangular meshes as in other members of the Dipteridinae and in such recent ferns as _Polypodium quercifolium_ (fig. 231, D, p. 297).

The imperfect lamina represented in fig. 289 may belong to _Hausmannia Richteri_ or may be a distinct species; it shows some of the finer veins connecting the shorter forked ribs, which formed part of the reticulate ramifying system in the mesophyll. This specimen was obtained from the plant-beds of Culgower on the Sutherlandshire coast, which have been placed by some geologists in the Kimmeridgian series.

The smaller type represented in fig. 278, E, is referred by Richter to a distinct species, _Hausmannia Sewardi_[977], founded on a few specimens from the Lower Cretaceous strata of Strohberg. This species is characterised by a stouter rhizome bearing smaller leaves consisting of a short petiole (3–4 cm. long) and an obovate lamina (1–2 cm. long and broad). There are usually two opposite leaflets on each leaf-stalk, and these may be equivalent to the two halves of a single deeply dissected lamina.

It is interesting to compare these different forms of _Hausmannia_ with the fronds of recent species of _Dipteris_ represented in fig. 231. The more deeply dissected type, such as _H. dichotoma_, closely resembles _D. Lobbiana_ or _D. quinquefurcata_, while the more or less entire fossil leaves (fig. 278, E, F and fig. 289) are very like the somewhat unusual form of _Dipteris conjugata_ shown in fig. 231, B, p. 297.

Other species of the genus are recorded from Liassic rocks of Steierdorf[978] (Hungary) and of Bornholm[979]. Nathorst[980] has described a small Rhaetic species from Scania: a French Permian plant described by Zeiller[981] and compared by him with _H. dichotoma_, may be a Palaeozoic example of this Dipteris-like genus.

Some segments of leaves from the Eocene beds (Middle Bagshot) of Bournemouth, and now in the British Museum, described by Gardner and Ettingshausen[982] as _Podoloma polypodioides_, bear a close resemblance in the venation to the lamina of _Dipteris conjugata_.