XXl] 



DIPTERIDINAE 



381 



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 appro.ximately at right angles and then sub- 

 divide into a reticulum, in the meshes of which the veinlets end blindly 

 (tig. 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 {Rliizoniopteris) is 

 creeping and dichotomously branched, bearing leaf-scars with a horse- 

 shoe form of vascular strand. 



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 con- 

 crescence 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. 



Pis. XIX. XX. 



1878. D. exile, Nathorst, Flora vid Bjuf, i. p. 39, PI. v. fig. 7. 

 — D. acutilobum, ibid. PI. xi. fig. 1. 



p. 77, 



