CONCHOLOGIA CESTRICA. 103 
Shell, heavy, oblique, inflated, anterior side longest, 
narrower, sub-angular ; beaks posterior, full, prominent, 
distant ; valves solid; striz regular, distinct; epidermis 
glossy, variable, straw, greenish, or yellowish, zoned; 
hinge margin elevated ; cardinals small, united; laterals 
strong, distinct, short. H. 4%, W. 5, T. 4, mill. 
Station, streams and rivulets. Chester County; 
abundant. 
P. abditum, HaLpEman, Proc. Acad. Nat. Sci., I., 1841. 
P. abditum. — {Prime.] 

Shell, round-oval, moderately convex, margins well 
rounded; beaks posterior, small; smooth, straw color; 
hinge margin nearly straight; cardinal teeth small, 
separate, the anterior larger, and more prominent; 
laterals small, not elongated. H. 3%, W. 4, T. 2, mill. 
Station, streams, fountains, and rivulets. Chester 
County ; frequent. 
