Fam. 16.] NATUKAL HISTOKY SOCIETY. 99 



lunule distinct. Animal with fringed mantle margins ; unequal, 

 somewhat separated siphons with fringed orifices. 



1. Venus mercenaria, Ij. (Figs. 5-7.) Hard Shell Clam. Quahog. 

 Round Clam. Shell thick, solid, obliquely ovate or heart-shaped; 

 lunule distinct, heart-shaped ; umbones directed forward and in- 

 ward; teeth three in each valve; surface covered with numer- 

 ous, laminated, concentric lines or ridges ; ligament large ; pallial 

 sinus acute, not very deep ; interior margin crenulated, especially 

 along the lower and front edges. Dirty or chalky white ; within 

 pure white with some bright violet near the distinct muscular 

 impressions. L. 75 ; H. 60 ; W. 50 ; U. ^. — Whole coast. Com- 

 mon, especially south of Mass. 



2. Gemma, Desh. 



Shell minute, rounded, smooth ; margin crenulated ; hinge 

 short, narrow ; cardinal teeth 2-3. 



1. Gemma purpurea, H, C. Lesi. Amethystine Gem. Shell mi- 

 nute, nearly round ; umbones slightly elevated, generally eroded ; 

 no defined lunule; muscular impressions and pallial line distinct; 

 the pallial sinus ascending, acute ; 3 cardinal teeth in left valve, 

 2 in the right; inner margin crenulated; surface shining, with 

 minute, concentric, crowded furrows. Y^ellowish, white or rosy, 

 with the umbones and posterior end usually amethyst color. 

 L. 4 ; H. S^; W. 2 ; U. ^. — Whole coast. Common on sandy 

 tide flats, just under the surface. 



Var Manhattensis, Prime. This variety is smaller, more tri- 

 angular, and more solid. White, straw-color to purplish. L, 3 ; 

 H. 3; W. IJ; U. J.— Mass. and south. Rare. 

 3. Cytherea, Lam. 



Shell similar to Venus, but the margins are not crenulated 

 and the cardinal teeth are 3 in one valve and 4 in the other. 



1. Cytherea convexa, Say. Very similar in form and parts to 

 Venus mercenaria, but smaller, with 4 cardinal teeth in the right 

 valve, no purple inside, and the edge not crenulated. L. 43 ; 

 H. 35 ; W. 24 ; U. J.— Whole coast. Rather rare. 

 4. DosiNiA, Scop. 



Shell orbicular, compressed, concentrically striated, pale ; liga- 

 ment sunk ; lunule deep ; hinge with 3-4 teeth. 



Animal with a hatchet-shaped foot projecting from the lower 

 side of the shell; siphons united to their ends. 



