yet it is readily distinguished from that shell, which hay 

 not a proper diaphragm, the labium being only a little 

 prominent and acute ; it is also a fresh water, operculated 

 shell, allied to Neritina. 



The genus Pileolus, Sowerby, has a very prominent lab- 

 ium or diaphragm, but the spire is altogether concealed. 



CREPIDULA PLANA. 



DESCRIPTION. 



Shell depressed, flat, oblong oval, transversely wrinkled, 

 lateral margins abruptly deflected ; apex not prominent 

 and constituting a mere terminal angle, obsolete in the 

 old shells ; within white ; diaphragm occupying half the 

 length of the shell, convex, at the edge contracted in the 

 middle and at one side. 



SYNONYM. 



C. PLANA, Nob. Journ, Acad. Nat. Sc, Vol. 2, p. 226. 



OBSERVATIONS. 



This species is remarkable for its depression, being with- 

 out convexity above, and is even often curved upward. 

 In order to give space beneath for the substance of the an- 

 imal, the margin of the shell is vertically deflected. The 

 general curvature is various in diflcrent individuals, being 

 modified in compliance with that of the surface on which 



