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.

Fig. 279.

Adiantides antiques (Ett.). (½ nat. size.)
A. Lindsayoides (Sew.). (B′ nat. size.)

(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.

Fig. 280. Onychiopsis Mantelli. (From Wealden specimens in the British Museum; No. 13495 and No. V. 2615. A, natural size; B, very slightly enlarged.)

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].”