238 Systematic Paleontology 



50 per cent larger than their fellows and more lobate. The usual type " 

 of pinnule is united to the rather slender rachis by a somewhat con- 

 stricted base. The proximal margin is usually entire, the apex rounded 

 and the distal margin has usually a single rounded lobe, separated from 

 the rounded apical lobe by a shallow sinus. The venation is fine and 

 sparse, and consists essentially of a single forked vein, one limb of which 

 runs to the apex of the pinnule and the other runs to the distal lobe. 

 The fertile pinnae are slightly reduced in size and more open than the 

 sterile, and bear an elliptical sorus at the apex of the vein which runs 

 to the distal lobe of the pinnule. 'No details of structure can be made 

 out. 



This species was described by Ward in 1895 as a species of Scleropteris 

 and compared with Scleropteris tenuisecta Saporta from the French 

 Kimmeridge. It resembles this species somewhat in general appearance, 

 and this is especially marked in the case of some of the more open speci- 

 mens with trilobate pinnules figured by Ward from Mt. Vernon. The 

 fertile specimens show conclusively, however, that this species is closely 

 allied to the modern forms of the tribe Dicksoniese of the family Cy- 

 atheacese. 



Professor Fontaine has described Dicksonia pachypJiylla from the 

 Shasta of California and Dicksonia montanensis from the Kootanie of 

 Montana, both of which have the lamina almost entirely reduced in the 

 fertile pinnse. 



The most nearly related fossil species is Dicksonia hellidula Heer ^ 

 from the Kome beds of Western Greenland. Curiously enough this was 

 originally described by Professor Heer as a species of Scleropteris," just 

 as Professor Ward made the same mistake in his identification of the 

 Potomac species. The Arctic species has shorter, broader, and more 

 rounded pinnules or pinnatifid lobes of the pinnae. It resembles the 

 Potomac species in having a single elliptical sorus at the apex of a vein 

 on the distal margin of each pinnule or pinnatifid lobe of a pinna. 



^ Heer, Fl. Foss. Arct., Band vi, Abth. ii, 1882, p. 1. 



2 Heer, Fl. Foss. Arct, Band iii, Abth. ii, 1874, p. 35, pi. ii, figs. 17c, d, 18; 

 pi. xi, fig. 8. 



