MARINE MOLLUSCA OP THE UNITED STATES. 193 



England coast and Gnlf of St. Lawrence, but lias probably been 

 transplanted to these localities. The lower valve is sometimes 

 ornamented with red or violet rays. 



2. 0. borealis, Larrfarck. Figs. 530, 531. 



Aniui. sans Vert., vii. 220. 1822. 



Shell rounded-ovate, the upper valve covered with membranous 

 scales, the lower valve irregularly spiny-ribbed and foliaceous. 

 Length 3 to 6 inches, breadth about 2 to 5 inches. 



New England, Neic York. 



This species is apparently very distinct from the preceding ; it 

 is smaller, wider, has not the lengthened beak of the lower valve 

 of Virginiana and the surface is much rougher. It is very closely 

 allied to 0. edulis of Europe. 



3. 0. triangularis, Holmes. 



Proc. Elliott Soc. Nat. Hist., 29. 1856. 

 Shell subtriangular, subequivalve, subequilateral, thick, lami- 

 nated ; beaks produced, acutely pointed, angular, and slightly 

 curved towards each other; margins rounded ; cavity of the shell 

 circular; muscular impression very large in proportion to the size 

 of the shell, and placed near the margin of the base. 



South Carolina. 



Dr. Holmes states that this shell resembles 0. edulis of Europe 

 but is more regular in form. Its large muscular impression, 

 pointed beaks, and triangular shape distinguish it from that 

 species. I am not acquainted with this shell. 



4. 0. equestris, Say. Figs. 532, 533. 



Am. Conch., vi. t. 58. 1834. 

 Shell small, ovate-triangular, more or less folded longitudinally; 

 lateral margins near the hinge with from six to twelve denticu- 

 lations of the superior valve received into corresponding cavities 

 of the inferior valve; superior valve depressed, but slightly folded ; 

 inferior valve convex, attached by a portion of its surface, the 

 margins elevated, folds unequal, much more profound than those 

 of the superior valve ; hinge very narrow, and curved laterally 

 and abruptly. 



North Carolina to West Indies. 

 13 



