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. [Illustration: FIG. 281. _Dictyophyllum exile._ (After Nathorst; much reduced.)] _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.