MARINE MOLLUSCA OF THE UNITED STATES. ~ 157 
or less united; foot linguiform, compressed, sometimes grooved ; 
palpi moderate, triangular, pointed ; branchiz 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, semioyate 
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. Dostntra. 
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. PETRICOLA. 
