CONCHOLOGIA CESTRICA, 93 
concentric, wrinkles, and radiated with green; beaks 
prominent, undulated, usually decorticated; the anterior 
portion of the valves thickened internally, and whitish; 
posteriorly, thin, and salmon color, with iridescence ; 
teeth stout, triangular, crenated, double in the left valve, 
and prolonged to the hinge margin. H. 1.6, W. 3, T. 
1.25, inches. 
Station, creeks and ponds. Chester County; common. 
M. marginata, Say. 
Alasmodonta marginata, Say, Jour. Acad. Nat. Sci., 
T:; 1818. 
M. marginata, 
Fig, 191, 

Shell, thin, transversely oblong, or sub-triangular, 
inflated, with the ligament slope abruptly depressed, 
and obliquely rugose; epidermis olive-brown, with 
green rays; within bluish-white; teeth compressed, 
oblique, nearly parallel with the anterior margin, and 
abrupt behind. H. 1.65, W. 2.8, T. 1.3, inches. 
Station, creeks and ponds. Chester County; frequent. 
Sub-genus ANODON (BrucirreE), CuvIER, 1798. 
Shell, equivalve, inequilateral, transverse; hinge 
margin linear, without teeth. 
Animal, mantle margin entirely free; the outer 
branchia united to the mantle, as far as its extremity ; 
the inner entirely united to the foot; foot thick, tongue- 
shaped, produced anteriorly. 
