GEOLOGICAL AFFINITIES OF THE rOTO.MAC I'LANTS 337 



(■'. coHstfida is voiy near to Alvtli(i))tcrls cycddina Sclioiik, (Vmii the AN'eal- 

 den. C. Vi)i)lnicims is perhaps not to be distiiig-uislied from IJroiigiiiart's 

 I'ccojjtcri^ Whilbicnsis of the ( )ohte of Kiighind, and is ver\' near to Airos- 

 tichkles limit nrfuViKs, from the lihatic of Vir;^inia. < '. tleiiliciilula is much 

 Wke Niiirupfc) is liijala, l^indley and Jlutton, from the ( )olite of Eiij^Iand. 

 C.falaitd is very simihir to a hirge nuniher of .Inrassie ferns grouped by 

 some xnuler the general name Wliithiensifi, beinj^;- in its smaller forms (jspe- 

 cially like 6'. «rMtv/s/.s Nathorst, from the lvh;etic of Sweden, ('.jxtridis 

 very elose to C. p-'icudowhithicnsis, from the Rlnetie of A'irginia. It is hardly 

 worth while, however, to trace further tlie resend)lance of the species of 

 Potomac CIadoi)hlebids to those of other floras. 



Pccopfcfia: The genus J'ccopfcris, taken simply as a t\pe of foliage 

 and nervation, has but little value in fixing- age, ftir this group does not 

 seem to be characteristic of any particular geological period. The genus 

 is but little developed in the Potomac, and its comparative rarity may 

 perhaps be taken as evidence, sucli as it is, of the comparatively recent 

 age of the beds. But in this genus we are enabled to detect two species 

 as connnon to the Potomac and known floras, while several are very close 

 to previously described plants. The identification of the Potomac frag- 

 ments with Ileer's 7*. Socialis, from the Atane beds or Middle Cretaceous 

 of Greenland, does not have much weight, as the Potomac fossils are (pn"te 

 rare and imperfect. The case is different with J'. Broivuiarta. This is an 

 important and characteristic plant in the Wealden of Europe. It is Avide- 

 spread in the Potomac beds, and is largely rei^resented by identical or 

 nearly allied forms, such as I'ccjijiteris strictinervis and P. constricta, giving', 

 indeed, a t\pe form. Besides these we have P. Virginiensis, nuich like /'. 

 (Icnficiildia Brong., an ( )olitic fossih 



Sphtmoptcris : The genus Sphoioptcris, like Pecoptcrifi, taken sim[)ly as 

 a type of foliage and nervation, does not characterize any particular era. 

 I)ut in the Wealden everywhere there is one particular iy\w oi SphLiioiyturis 

 which is highly characteristic of that formation. This is the so-called 

 species Sphvnoptcris Maiitelli. The adjective so-called is used because in 

 my opinion too many probably' different j^lants have been united under 

 this name, as has been done in the case of Pecopteris Whithicnsls. As 

 MON XV 22 



