OF THE UNITED STATES. 309 



MACTRA, Lam. 

 Species. 



1. M. *similis. Shell subtrigonate, smooth, or 

 very slightly wrinkled, white on the disk or upon 

 the umbones, and dirty light brownish colour on the 

 margin ; umbones nearly central ; lateral teeth strong- 

 ly and regularly crenated on the side next the reci- 

 pient cavity. 



Length one inch and three-twentieths, nearly. 



Width one inch and two-fifths. 



Inhabits the coast of the United States. 



Cabinet of the Academy. 



The specimens which 1 obtained from the coast of 

 New Jersey seem closely allied to Var. a. of M. 

 solida as presented on pi. 258, fis;. 1. of the Encyc. 

 Method., the proportions of the different parts of the 

 shell nearly corresponding with those of that figure. 

 But upon comparing it with several specimens of 

 M. solida sent to the Academy by Mr. O'Kelly of 

 Ireland, I find its proportions to be altogether dif- 

 ferent, being longer and not so wide. 



2. M. ^lateralis. Shell triangular, very convex, 

 of a smooth appearance, but with very minute, trans- 

 verse wrinkles ; lateral margins flattened, cordate, 

 with a rectilinear, sometimes concave profile, one 

 margin rounded at the tip, the other longer and less 

 obtuse 5 umbo nearly central, prominent. 



