CONCHOLOGIA CESTRICA. IOI 
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; striz 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. 
Pisidium compressum, — [Prime.] 
Fig, 204, 

9* 
