MARINE MOLLUSCA OF THE UNITED STATES. 145 



mingle, I have satisfied myself 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 transverse 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. 

 Mactra polynyma, Stimpson, Shells ofN. Eng. 20. 1851. 



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

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

 teeth not striated; sinus of pallial impression deep. 



New 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 young of this species, described by Stimpson as M. polynyma. 



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



Journ. Philad. Acad. ii., 309. 1822. 



Shell triangular, ver} r 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 ; urnbo nearly 

 central, prominent. 

 .Length 22, height 20, diam. 15 mill. 



Whole Coatt. 



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. 



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

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



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

 margin; uinbo hardly prominent, from which a carinated angle 

 passes to the posterior extremity ; epidermis pale brownish-yellow, 

 tinged with ferruginous ; within white, highly polished. 



North Carolina to West Indies. 



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