MARINE MOLLUSCA OP THE UNITED STATES. 173 



smooth and polished, broad anteriorly and sloping at the posterior 

 dorsal margin; beaks elevated; and lower margin nearly straight. 

 Animal white, nearly transparent ; mantle extending beyond the 

 margin of the shell, open in front, with undulated but unfringed 

 margins; foot large and powerful and may be expanded to double 

 the length of the shell, with a posterior heel. The animal suspends 

 itself by its foot, or can use it for creeping like a gasteropod. 



Charleston, 8. C. 



Unidentified Species. 



3. L. FABAGELLA, Conrad. Figs. 442-444. 



Am. Mar. Conch., 53, t. xi. f. 3. 



Shell suboval, convex, with minute crowded concentric lines ; 

 beaks central, rather prominent; epidermis yellowish, very thin, 

 wrinkled ; teeth similar in each valve; the posterior tooth longest, 

 and angulated under the beak. 



Rhode^Island. 



A single specimen obtained. 



Family CARDIID^. 



Palpi slender, acuminate. Mantle freely open in front; siphons 

 distinct but very short, and nearly sessile, their bases and sides 

 furnished with tentacular filaments ; gills two on each side, thick, 

 united together behind the body. Foot very long and geniculate. 



Synopsis of Genera. 



Shell globose, gibbose, nearly equilateral, more or less gaping posteriorly! 

 the margins often serrated ; valves with elevated radiating ribs. 



CARDIUM. 



Shell longitudinally.oval, inequilateral ; surface of valves plain (not ribbed); 



hinder gap small. L^VICABDIUM. 



Shell subcordate, compressed, rather thin, subequilateral, valves with 



obsolete, radiating ridges, sliglitly gaping; beaks rather prominent. 



Hinge with the cardinal teeth wanting. SERRIPES, Beck. 



Genus CAEDITJM, Linnaeus. 

 Syst. Nat, edit. x. 1758. 



1. C. ISOCARDIA, Linnaeus. Fig. 445. 



Syst. Nat., edit. x. 679. 1758. 

 Shell obliquely heart-shaped, gibbous; radiately ribbed, ribs 



