53 



CARDIUM. 



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, 



