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., 

 I., 1818. 



Jtf. marginata. 



Pig, 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 (BRUGIERE), 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. 



