194 CARDIIDA. 
the species, we find the following statement: ‘‘a genus which 
became extinct in the upper cretaceous period.” 
Apuropits, Lea, 1834. 
Syn.—Acardo, Swains. [pt.], 1840. Serripes, Beck, 1844. 
Distr.—A. Grenlandica, Chemn. (exvi, 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. LZ. serratum, Linn. (exvi, 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. JL. 
HMillanum, Sowb., is a cretaceous example. 
NEMOCARDIUM, Meek, 1876. Shell closely resembling the typical 
forms of Protocardia, but thinner, with two-thirds to three- 
fourths of surface in front of the stronger posterior, usually 
echinate, radiating costs, occupied by fine, crowded, radiating 
striae, and the free margins crenate within a around ; cardinal 
and lateral teeth generally rather slender; pallial line faintly 
sinuous, irregularly serrated, or nearly simple behind. L. semias- 
perum, Desh. 
PACHYCARDIUM, Conr., 1870. Shell very massive, much higher 
than long; valves very gibbous; hinge remarkably strong ; 
surface sculpturing rather obscure, the posterior radiating cost 
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 costz being obsolete, or often only indicated by 
crenulations along the posterior third of the free margins 
within; pallial line with two shallowsinuses. JL. subquadratum, 
Evans and Shumard. 
Apacna, Hichwald, 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 
inmud. A. edentulum, Pallas (exvi, 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. 
