DESCRIPTION OF THE SrECIKS. 155 



Saporta calls attention to the resemblance of the Jurassic ctenopterids 

 to the odontopterids of tlie Carboniferous, and says that there is much 

 difficulty in giving any good distinction between C/ennptcris and Zigno's 

 genus Dichopteris from the Oolite. He thinks it possible that Dichoptcy'is 

 may represent Ctcnoptcris in the Oolite, the main difference being that 

 Dichopteris has its pinnules narrowed at base. Saporta also remarks that 

 Sdcroptcris is near to Ctcnopteris. The Potomac plants bring these genera 

 still nearer, supplying connecting links, for we find some with the charac- 

 ters of each of these genera predominating, and they graduate into one 

 another. They show that the features relied upon to distinguish Sdcrop- 

 tcris, Ctcnopteris, and Biclioptcris have in the main but little value. 

 Scleropteris may remain a distinct genus, but, as has been suggested, there 

 seems to be no good reason why Dichopteris should not be united to 

 Clenopteris; for, as Saporta says, the bifurcation of the frond of the former 

 may have been accidental. If we omit the slight union of the pinnules 

 at base and the absence of teeth, the resemblance between the nervation 

 of Dichopteris Visianica,^ its shape, and the attachment of the pinnules, and 

 these features in some of the Potomac ctenopterids is most striking. AVo 

 have the same slight forward curvature of the pinnules, the oblique cutting 

 away in front, the sliglit decurrence below, and the same mode of insertion 

 of the nerves. The nerves differ only in tixe more copious branching of 

 the central ones of the Potomac plants. 



Ileer^ gives figures of plants which he places in his cycadeau genus 

 Cienidium. These, and more especially the species C. dentatum, resemble 

 in some points the Potomac plants; but in Heer's plants the cycadean feat- 

 ures predominate, and they certainly belong to a genus distinct from that 

 of the Potomac fossils. Phillips's Odontopterls Lcckenbyi Zigno (Geol. of 

 Yorkshire, p. 218) may also be compared with these plants. The species 

 Ctcnopteris Iticri^ Sap. seems to belong to a type quite different from C. 

 grandis and C. cycadea, having the nervation only in common. C. Iticri 

 is much more fern-like than the two latter. 



'Zigno: Flor. Foss. Form. Oolitica", PI. XII. 



«Flor. Foss. (la Portugal, PI. XVI. 



'Pal. Franfaise, Plautes Jar., vol. 1, Atla.s, PI. XLIV, Figs. 1, :i. 



