MARINE MOLLUSCA OF THE UNITED STATES. 157 



or less united ; foot linguiform, compressed, sometimes grooved ; 

 palpi moderate, triangular, pointed ; branchiae large, subquadrate' 

 united posteriorly. 



Synopsis of Genera. 



Shell ventricose, subglobose, triangularly heart-shaped ; valves with the 

 margins finely crenulated. Hinge with three erect compressed diverging 

 teeth in each valve, the anterior in the left and posterior in the right valve 

 strong and somewhat bifid, the others simple and lamellar ; lozenge pro- 

 minent, obliquely sulcately crenate within. Pallial line remote from the 

 ventral margin, ending behind in a short, narrow, triangular sinus. 



MERCENARIA. 



Shell roundly triangular, subequilateral ; surface of valves smooth, the 

 margins crenulated. Three primary teeth in the left valve, the middle 

 one conical and slightly arched, and two in the right valve, diverging, 

 with a wide pit between them. Muscular impressions ventral ; pallial 

 line .marginal, with a very long, narrow, deep sinus ascending perpendi- 

 cularly. GEMMA. 



Shell ovately triangular, more or less thickened or subcordiform ; margins 

 of the valves finely crenulated. Hinge narrow, solid, tridentate in the 

 right valve, bidentate in the left ; teeth divaricate, unequal, the anterior 

 tooth the longest. Pallial line with the sinus nearly obsolete, or very 

 short and triangular. CHIONE. 



Shell transverse, ovate, inequilateral ; margins of the valves entire, often 

 obtuse. Hinge tridentate in the left valve, the anterior lateral tooth 

 united to the lunular tooth. Pallial line with a wide, deep, semiovate 

 sinus. CALLISTA. 



Shell orbicular, compressed, concentrically striated, deeply lunulate 

 under the beaks. Hinge with three teeth in each valve, the lunular 

 tooth elongate and compressed ; ligament external, partially concealed 

 under the lozenge. Sinus of pallial impression deep, oblique, triangular, 

 with the apex acuminate. . DOSINIA. 



Shell transversely ovate, inequilateral, margins entire. Hinge tridentate, 

 teeth sometimes diverging, sometimes approximate, subparallel, often 

 bifid or canaliculate at the apex. Pallial impression deeply sinuated 

 posteriorly ; sinus semioval, somewhat horizontal. TAPES. 



Shell oblong or ovate, white, covered with a hard, thin epidermis, ventri- 

 cose, anterior side short, posterior gaping. Hinge composed of two pri- 

 mary teeth in each valve, one of which is often obsolete or rudimentary; 

 lateral teeth none; ligament short, external. Pallial line with a deep 

 rounded sinus. PETEICOLA. 



