﻿L8 



EOCENE MOLLUSCA. 



adopted that I did not then and do not now consider it necessary to alter it ; and I have 

 therefore retained the name of Cyclas, following thus the example of several other 

 modern conchologists. This has been justly separated from Cyrena, which is a thick 

 and strong shell. Another division has been proposed under the name of Pisidium for 

 some freshwater Bivalves very closely resembling those of Cyclas, being thin and semi- 

 pellucid in the living state, like Cyclas, but having the shell a trifle more inequilateral, and 

 presenting some difference in the siphon. Some of the Lower Tertiary fossils have been 

 figured and described under each of these generic names, but those which I have seen 

 may, I think, be referred to Cyclas. 



1. Cyclas Bristovii ? Forbes. Tab. B, fig. 5. 



Cyclas Bristovii, Forbes. Mem. Geol. Surv. Isle of Wight, p. 146, pi. xi, fig. 3, 1856. 



— — Morris. Catal. Brit. Foss., 2nd edit., p. 198, 1854. 



— — J. Lowry. Chart. Brit. Tert. Foss., pi. ii, 1866. 



" A small, somewhat quadrate and rather ventricose shell, the anterior margin rounded 

 and the posterior truncated ; the surface is concentrically marked by the fine lines of 

 growth. This species presents considerable resemblance to a form from Headon Hill in 

 Mr. Edwards's Cabinet." — Morris. 



Length, |4h of an inch ; height, \t\\ of an inch. 



Locality. Hempstead {Morris). 



A small shell apparently belonging to the genus Cyclas has been figured as above 

 referred to, but it is insufficient for full description or comparison. A similar specimen is 

 in my own cabinet, but, as it rests upon a piece of clay with its back uppermost, it cannot 

 be satisfactory described. Mr. Morris, in the 1 Geo. Survey Memoir,' has not given the 

 characters of the hinge of the interior, so that some uncertainty attaches to his 

 species. 



M. Deshayes has figured and described three species with this generic name, and 

 four with that of Pisidium. These are all said to be from Lignites or Sables inferieurs 

 of France, but, judging from figures and descriptions, I cannot refer our shell to any one 

 of his species. 



Mr. Whitaker in his report upon the Woolwich Beds (p. 577), gives the name of 

 " 'Cyclas? (a small Bivalve) " from Chiselhurst, but he informs me that he is unable to 

 refer me to the specimen, and that it may be disregarded. 



