18 EOCENE MOLLUSCA. 
adopted that I did not then and do not now consider it necessary to alter it; and I have 
therefore retamed 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 Pésidiwm 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. Cycnas Bristovir ? Forbes. Tab. B, fig. 5. 
Cycias Brisrovu, Forbes. Mem. Geol. Surv. Isle of Wight, p. 146, pl. xi, fig. 3, 1856. 
— — Morris. Catal. Brit. Foss., 2nd edit., p. 198, 1854. 
—_ — J. Lowry. Chart. Brit. Tert. Foss., pl. 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, sth of an inch ; height, }th of an inch. 
Locality. Wempstead (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 ‘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 Pstdiwm. These are all said to be from Lignites or Sables inférieurs 
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. 
