TO GEOLOGY. 67 



the first to collect and arrange systematically the shells of 

 our country, he has formed one of the finest cabinets in it. 



C. subcrassa. Plate 2. Fig. 43. 



Description. Shell somewhat inflated, subtriangular, 

 concentrically striate, thick about the region of the hinge ; 

 substance of the shell thick ; beaks thick and elevated ; 

 lunule cordate, indistinct ; teeth rather elevated and com- 

 pressed ; excavation of the palleal impression rather small 

 and roundish ; cavity of the shell deep and rounded ; mar- 

 gin crenulate. 



Diam. . , . . Length 1.1, Breadth 1.2, of an inch. 



Observations. This shell most resembles the trigoniata 

 herein described. It is like that shell in outline, but is 

 thicker, and differs also in having a crenulated margin. 



C. trigoniata. Plate 2. Fig. 44. 



Description. Shell somewhat inflated, triangular, con- 

 centrically and minutely striate ; substance of the shell 

 rather thick ; beaks moderately elevated and recurved ; 

 lunule long, elliptical ; teeth moderately large ; excavation 

 of the palleal impression deep and rounded ; cavity of the 

 shell deep, subangular ; margin entire. 



Diam. .6, Length .9, Breadth 1.1, of an inch. 



Observations. In outline the trigoniata resembles closely 

 the subcrassa. It is, however, less thick in the substance 

 of the shell, has much finer striae, and is entirely without 

 crenulations on the margin. 



