CONCHOLOGIA, 



S. securis, PRIME. 



Cyclas securis, Prime, Bost Proc., IV., 1851 



S. securis. [Prime.] 

 Fig. 203. 



Shell, thin, rhombic-orbicular, ventricose, subequi- 

 lateral ; margins rounded, basal rather abrupt ; beaks 

 large, calyculate, approximate, inclined forwards ; valves 

 very convex ; striae regular, delicate ; greenish or yellow, 

 glossy ; cardinal teeth very small, laterals slight, nar- 

 row, elongated. H. 7^, W. 9, T. 6, mill. 



Station, ditches and running streams, tributaries of 

 Pickering Creek. Chester County ; abundant. 



Genus PISIDIUM, C. PFEIFFER, 1821. 



Shell, small, round-oval, inequilateral, anterior side 

 longer ; beaks terminal ; cardinal teeth double, or united ; 

 laterals elongated, double in the right valve, single in 

 the left ; ligament on the shorter side. 



Animal, lobes of the mantle united posteriorly into a 

 single short siphon ; oral tentacles triangular, elongated ; 

 foot small, tongue-shaped, very extensible. 



P. compressum, PRIME, Bost. Proc., IV., 1851. 



JPisidiutn compressum. [Prime.] 

 Fig, 204. 



