o3 
C A R D I U M. 
A. Compressed. (Plate V. Fig. 7.) 
A. Sub-equilateral and turgid. (Fig. 6.) 
Shell sub-equilateral, mostly equivalve, convex, 
longitudinally ribbed, striated or channelled, mar- 
gin toothed. Hinge with two primary teeth al- 
ternate, both much incurvated, lateral ones remote 
and inserted in the opposite valve. The beaks 
are turned inward, and the bosses turgid. — Many 
of the species in this genus bear the appearance 
of a heart, when viewed with either of the slopes 
in front ; some very exactly assume, and all more 
or less approximate to, the cordate form. 
One obvious character of the Cardium is, that 
the ribs are usually if not universally longitu- 
dinal, and not concentric or transverse, as they 
chiefly are in Tellina and Venus, which approach 
most nearly to the same heart-like outline, and 
are therefore in their exterior appearance most 
easily confounded with the Cardium. The disk 
is usually convex, but sometimes much compressed. 
