Shell equivalve, inequilateral, sub-orbicular, sides generally with longitudinal rayed ribs; hinge with two oblique cardinal teeth in each valve, turned in the same direction.
V. imbricata. The imbricated Venericardia. Pl. [9], fig. 1.
Species almost round, having convex longitudinal ribs, covered with imbricated rough scales; inferior margin rounded and dentated; more and more equilateral; the two teeth short and oblique.
FAMILY X.
Cardiacea. Five genera.
1. Cardium. Cockle, or Heart Shell. Forty-eight species.
This genus received its name from its resemblance to a heart (καρδια). It is so well defined by Linnæus that no alteration was made by Lamarck, except in making two divisions of them; the first distinguished by having the anterior side as large or larger than the posterior, and no distinct angle at the apex; the second by possessing carinated or angular umbones, and the posterior side often much larger than the anterior. These shells are found at a small depth in the sand on almost every seashore.
Shell inflated, equivalve, sub-cordiform (when seen anteriorly), generally costated from the summit to the circumference; summits very evident; slightly recurved forward; hinge complex, similar, formed by two oblique cardinal teeth, articulating with the corresponding teeth on the other valve; two distant lateral teeth on each valve; ligament dorsal, posterior, and very short.
Cardium costatum.
C. Indicum.
C. ringens.