MARINE MOLLUSCA OF THE UNITED STATES, 145 



mingle, I have satisfied mj'self that similis is not distinct ; but I 

 figure the latter species (Fig. 337) so that collectors can form 

 their own conclusions. M. Eaveneli is a more ti'ansverse variety, 

 but does not seem to possess any distinctive characters. 



This is the largest of our bivalve shells, attaining, in northern 

 specimens, a length of seven inches, and great solidity. 



2. M. ovALis, Gould. Figs. 340, 341. 



Invert. Mass., edit. i. 53. 1841. 

 Maetra polynyma, Stimpson, Sliells-ol^.En^T-S^l. ^1:651. S-0- ^^t-c^U^^ ' l--. 



Shell large, thick, obovate, coarse, nearly equipartite, covered 

 with a tough, dusky-brown epidermis ; V tooth strong, lateral 

 teeth not striated ; sinus of pallial impression deep. 



Neio England to N. Carolina. 



This species is readily distinguished from solidissima by its 

 shorter, more ventricose form, and the smooth lateral teeth. It 

 is somewhat smaller in size, yet becomes quite ponderous with 

 age, attaining a length of over four inches. Fig. 341 represents 

 the 3^oung of this species, described by Stimpson as 31. polynyma. 



3. M. LATERALIS, Say. Figs. 338, 339. 



Journ. Philad. Acad, ii., 309. 1832. 

 Shell triangular, verj' convex, of a smooth appearance, but with 

 minute concentric wrinkles ; lateral margins flattened cordate, 

 with a rectilinear, sometimes concave profile ; one margin rounded 

 at the extremity, the other longer and less obtuse ; umljo nearl}'- 

 central, prominent. 



Length 22, height 20, diara. 1.5 mill. 



Whole Coast. 



This is more triangular and ventricose than M. ovalis : the 

 flexuous and obtusely ridged posterior margin also distinguishes it. 



4. M. FRAGiLis, Chemnitz, Fig. 342. 



Coucli. Cab., vi. 236, t. 24, f. 235. 

 Mactra oMonga, Say. Journ. Philad. Acad., ii. 310. 1822. 



Shell oblong-oval, very slightly wrinkled, excepting upon the 

 margin ; umbo hardly prominent, from which a carinated angle 

 passes to the posterior extreniit}'; epidermis pale brownish-yellow, 

 tinged with ferruginous ; within wliite, liigidy polished. 



Worth Carolina to West Indies. 

 10 



