MARINE MOLLUSCA OF 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 r 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, S. C. 



Unidentified Species. 



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

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

 Shell suboval, convex, with minute ciwvded 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. 



RTiode\Idtcnd. 



A single specimen obtained. 



Family CARDIID^E. 



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

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

 fhrnished 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^evicardium. 



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



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



Hinge with the cardinal teeth wanting. ' Serripes, Beck. 



Genus CARDIUM, Linnteus. 

 Syst. Nat., edit. x. 1758. 



1. C. isocardia, Linnaeus. Fig. 445. 

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

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



