In describing some Belgian impressions of Devonian age as Lepidodendron gaspianum Daws. Crépin[72] states that Carruthers has come to regard the specimens named by him Psilophyton Dechianum as branches of a Lepidodendron; he also quotes Carruthers as having expressed the opinion that the name Psilophyton had been employed by Dawson for two kinds of fossils, some being twigs of Lepidodendron while others, identified by Dawson as the reproductive branches of species of Psilophyton, represent the spore-cases of ferns comparable with Stur’s genus Rhodea[73]. One of the examples figured by Carruthers[74] as P. Dechianum from Thurso (preserved in the British Museum, no. 52636), measuring 34 cm. in length and 8 mm. broad, bears a close resemblance to a fern rhizome covered with ramental scales such as that of a species of Davallia. Other Belgian specimens described by Gilkinet[75] as Lepidodendron burnotense, like Crépin’s species, are no doubt generically identical with some of the Scotch and Canadian fossils placed in the genus Psilophyton, though Penhallow[76] considers that the species Lycopodites Milleri is more correctly referred to Lycopodites than to Psilophyton.
A more recent paper on the Geology of the Perry basin in South-eastern Maine by Smith and White[77] contains a critical summary of the literature on Psilophyton and drawings of specimens. The latter afford good examples of Pre-Carboniferous plant fragments, such as are often met with in various parts of the world, which conform in habit to the New Brunswick specimens made by Dawson the type of his genus.
An examination of material in the Montreal Museum and of Hugh Miller’s specimens in the Edinburgh collection leads me to share the opinion of Count Solms-Laubach that the name Psilophyton has been applied to plants which should not be included under one generic title. As Kidston[78] pointed out, the Canadian species Psilophyton robustius is not generically distinct from British and Belgian specimens referred to Lepidodendron; it may possibly be identical with the Bohemian plants on which Stur founded his genus Hostinella[79]. The Devonian plants described by Stur have since been examined by Jahn[80] who regards them as vascular plants, and not as algae to which Stur referred them; he mentions two species of Psilophyton but gives no figures.
The “spore-cases” of Dawson may be found to be the microsporangia or perhaps the small seeds of some pteridosperm; the forked axes with a smooth surface and others figured by Miller and by Dawson, with the surface covered with scales suggesting the ramenta of a fern, may be the rachises or rhizomes of filicinean plants. Other specimens may be Lepidodendron twigs, as for example the petrified fragments figured by Dawson as Psilophyton princeps; while the stem identified as P. robustius is most probably that of a Gymnosperm. It is doubtful whether a useful purpose is served by retaining the genus Psilophyton. It was in the first instance instituted on the assumption, which cannot be upheld, that the abundant material in the New Brunswick beds bore a sufficiently close resemblance to the rhizome and aerial branches of Psilotum. Psilophyton has served as a name for miscellaneous plant fragments, many of which are indeterminable. Dr White concludes his account of the genus with the following words[81]:
“The examination of such so-called Psilophyton material as I have seen shows the existence in America of two or more groups, represented by several fairly well-marked species which possess stratigraphical value, and which should be carefully diagnosed and illustrated. It is probable also that additional material throwing light on the structure and relationships of these very remarkable early types of land-plants will be discovered at some locality. The inspection of the material in hand emphasises the need, as was pointed out by Solms-Laubach, for the revision of the material referred by various authors to Psilophyton, together with a thorough re-examination and re-publication of the types.”
Until a thorough re-examination has been made of the Canadian material, with a view to determine whether there exist substantial reasons for the retention of Dawson’s genus, it is undesirable to continue to make use of this name for Pre-Carboniferous fossils which are too incomplete to be assigned with certainty to a definite group of plants. Dr White draws attention to the similarity of some of the Perry basin specimens to Nathorst’s genus Cephalotheca[82] from Devonian rocks of Bear Island in the Arctic regions, a comparison which might be extended to other genera and which serves to illustrate the possibility that many of the specimens labelled Psilophyton may eventually be recognised as examples of well defined generic types belonging to more than one group of plants.
CHAPTER XIV.
LYCOPODIALES.
The recent members of the Lycopodiales are considered apart from the extinct genera in order that our examination of the latter may be facilitated by a knowledge of the salient characteristics of the surviving types of this important section of the Pteridophyta. A general acquaintance with the extinct as well as with the recent genera will enable us to appreciate the contrasts between the living and the fossil forms and to realise the prominent position occupied by this group in the Palaeozoic period, a position in striking contrast to the part played by the diminutive survivors in the vegetation of the present day. In the account of the recent genera special attention is drawn to such features as afford a clue to the interpretation of the fossils, and the point of view adopted, which at times may appear to lead to an excessive attention to details, is necessarily somewhat different from that represented in botanical text-books[83].