348 Systematic Paleontology 



intermediate position of tlie pinnae, linear in outline. Apex unknown, 

 analogy would suggest that it was obtuse. Base slightly enlarged above 

 and decurrent below, two adjacent segments subtending a rounded, 

 parallel-sided sinus, the width of the sinus being about half that of a 

 segment. Texture coriaceous. The veins branch from the rachis 

 at regular intervals at acute, approximately parallel angles, bending 

 outward almost immediately and usually forking, the subordinate veins 

 running close together, and parallel. The members of a single pair are 

 about 0.333 mm. apart, while the space between each pair is about 

 1 mm. Occasionally some of the veins fork again some distance from 

 the rachis the branches running parallel with the remaining veins. 

 The veins are slender and sharply defined when seen on specimens show- 

 ing the lower surface of the fronds or on impressions of the lower surface, 

 but they appear as a single flat band on impressions of the upper surface. 



With the genera Ctenis, Gtenidium, Otenopteris, Ctenozamites and 

 Ctenophyllum already in the field it might seem unnecessary to propose 

 a new genus for the fronds of this general type, and yet the Pofomac 

 species cannot be forced into any of these genera without unduly extend- 

 ing their limits. Described originally as a species of Ctenophyllum it 

 differs from that genus in the character of the venation which is strictly 

 simple and parallel in the latter. Ctenophyllum is moreover an older 

 Mesozoic genus first known in the Triassic and culminating in the middle' 

 Jurassic. The type and the bulk of the species are late Triassic and 

 none are known above the Oolite. The form in hand greatly resembles 

 Ctenis in habit but the veins as far as observed do not anastomose as in 

 that genus. The genera Ctenidium and Ctenopteris are quite different 

 from the present genus in all of their characters, as is Ctenozamites. 

 The two latter genera are bi- or tripinnate and the former has simple 

 veins. 



Ctenopsis may be looked upon as a gerontic type embracing some of 

 the characters of both Ctenis and Ctenophyllum and representing one 

 of the last authentic occurrences of this general type of cycad frond. 



While the foregoing discussion was going through the press, Seward 

 described a new frond genus from the Jurassic of Scotland as Pseu- 



