122 



should reveal a specific difference, I would suggest, for the east- 

 ern shells, the name P. elongatinay 



remarks on cyclas and lymnea. by dr. james lewis. 

 Cyclas. 



The genus Cyclas may be divided into three groups, having 

 the following distinguishing features : — 



First Group. Shells robust and somewhat ponderous ; sur- 

 face usually marked with strong ridges, which, in some species, 

 assume a very regular development on the beaks, or embryonic 

 portion of the shell ; epidermis rather coarse, and often of a dark 

 color, though this feature is liable to considerable change in the 

 same species. 



Examples. C. similis, C. solidula^ C.fuscala, C. distorta, &c. 



Second Group. Shells less robust, but moderately firm ; sur- 

 face generally smooth, though the lines of growth are usually 

 somewhat conspicuous ; valves usually inflated ; epidermis finer 

 than in the preceding group, and often marked by zones of 

 strongly contrasting.colors. 



Examples. C. elegans^ C. occidentalism C. cardissa, &c. 



Third Group. Shells thin and fragile, and often translucent ; 

 beaks usually elevated above the hinge margin, in a conspicuous 

 manner, and surrounded by the embryonic portion of the shell, 

 which is distinctly separated from the rest of the shell, by a 

 deeply indented line ; surface of the shell very smooth, and the 

 lines of growth frequently so fine as not to be easily distinguished ; 

 epidermis fine, and sometimes zoned by delicate shades, but not 

 often by strongly contrasting colors. 



Examples. C. transversa, C. partumeia, C. securis, &c. 



Lymnea. 



In an article on Lymnea, that I presented to the Society a 

 few months since, some remarks were offered, showing the spe- 

 cific identity of two forms of Lymnea found in contiguous local- 

 iiies. The forms previously alluded to are, first, a variety 

 recognized as one of the forms of L. catascopium^ Say ; second, 

 a variety which some of my correspondents recognize as L. 

 emarginata, Say. 



