CONCHOLOGIA CESTRICA. opt 
rounded, extremity ; hinge margin elevated; base slightly 
curved; posteriorly sloped above, and below, to an acute 
angle; dark brown, or greenish; nacre hyaline-purple ; 
cardinal teeth small, compressed, oblique. H. .85, W. 
225, Laayimches. 
Station, White Clay Creek, Chester County. 
U. heterodon, Lea, Trans. Amer. Phil. Soct., IIT., 1829. 
U. heterodon. 
Fig. 188, 

Shell, small, thin, greenish-brown, obscurely rayed; 
rhomboid-ovate, ventricose; rounded before; base nearly 
straight; dorsal margin elevated, behind the hinge; 
posterior margin oblique; beaks undulated, prominent; 
nacre bluish-white; cardinal teeth compressed, oblique; 
lateral tooth double in the right valve. H..5, W. .8, T. 
.4, inch. 
Station, Schuylkill River, Chester County; abundant. 
Sub-genus MARGARITANA, ScHUMACHER, 1817. 
Shell, thick, strong; cardinal teeth stout, triangular ; 
lateral tooth absent. 
Animal, gills free from the abdominal sac, and not 
united to the mantle, posteriorly; inferior siphon 
fringed, superior plane. 
M. margaritifera, LINN. 
Mya margaritifera, Linnaeus, Syst. Nat., 1758. 
Alasmodonta, Barnes, 1823. 
