194 CARDIIDJE. 



the species, we find the following statement: " a genus which 

 became extinct in the upper cretaceous period." 



Aphrodita, Lea, 1834. 

 Syn. — Acardo, Swains, [pt.], 1840. Serripes, Beck, 1844. 

 Distr. — A. Grctnlandica, Chemn. (cxvi, 79-81). Arctic seas. 

 Shell subcordiform, compressed, subequilateral; surface 



smooth or slightly radiately striate ; beaks prominent ; cardinal 



and lateral teeth obtuse, small, almost obsolete. 



L^viCARDiUM, Swainson, 1840. 



Syn. — Liocardium, Morch, 1852. 



Distr. — 21 sp. • Universal. L. serratum, Linn, (cxvi, 83). 



Shell oval, elongated, oblique, somewhat inequilateral ; surface 

 smooth or lightly radiately striate. 



PROTOCARDiUM, Beyrich, 1845. Posterior half of shell radiately 

 striate, anteriorly half distantly concentrically striate. L. lyra- 

 tum, Sowb. (cxvi, 82). There are several recent forms. L. 

 Hillanwrn^ Sowb., is a cretaceous example. 



NEMOCARDiUM, Meek, 1876. Shell closely resembling the typical 

 forms of Protocardia, but thinner, with two-thirds to three- 

 foui-ths of surface in front of the stronger posterior, usually 

 echinate, radiating costse, occupied b}^ fine, crowded, radiating 

 striae, and the free margins crenate within all around ; cardinal 

 and latei'al teeth genei'ally rather slender ; pallial line faintly 

 sinuous, irregularly serrated, or nearly simple behind. L. semias- 

 perum, Desh. 



PACHYCARDTUM, Conr., 1870. Shell very massive, much higher 

 than long ; valves very gibbous ; hinge remarkably strong ; 

 surface sculpturing rather obscure, the posterior radiating costse 

 being nearly obsolete. L. Spillmani, Conr. 



LEPTOCARDIA, Meek, 1876. Shell small, very thin, as high or 

 higher than long; hinge weak; surface nearly smooth, the pos- 

 terior radiating costse being obsolete, or often only indicated by 

 crenulations along the posterior third of the free margins 

 within ; pallial line with two shallow sinuses, L. suhquadratum, 

 Evans and Shumard, 



Adacna, Eichwald, 1838. 



Syn. — Acardo, Swains, [pt.], not Brug. Hypanis, Pander. 



Distr. — 8 sp. Aral, Caspian, Azof, Black Sea, and the em- 

 bouchures of the Wolga, Dniester, Dnieper, and Don; burrowing 

 in mud. A. edentulum, Pallas (cxvi, 84). 



Shell compressed, gaping behind, thin, nearly edentulous ; 

 pallial line sinuated. 



Animal with the foot compressed; siphons elongated, united 

 nearly to the end, plain. The siphonal inflection varies in amount. 



