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. J/. Raveneli 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. ovauis, Gould. Figs. 340, 341. 
Invert. Mass., edit. i. 58. 1841. 
Mactra polynyma, Stimpson, Shells of N. Eng. 20. 1851. 
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. 
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 JIL. polynyma. 
3. M. LATERALIS, Say. Figs. 338, 339. 
Journ. Philad. Acad. ii., 309. 1822. 
Shell triangular, very 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; umbo nearly 
central, prominent. 
Length 22, height 20, diam. 15 mill. 
Whole Coast. 
This is more triangular and ventricose than J. ovalis; the 
flexuous and obtusely ridged posterior margin also distinguishes it. 
4. M. rraciuis, Chemnitz. Fig. 342. 
Conch. 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; umbo hardly prominent, from which a ecarinated angle 
passes to the posterior extremity ; epidermis pale brownish-yellow, 
tinged with ferruginous ; within white, highly polished. 
North Carolina to West Indies. 
10 . 
