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.
Fig. 281. Dictyophyllum exile. (After Nathorst; much reduced.)