BIVALVIA. 141 
CARDITA. Bruguiére, 1789. 
Generic Character. Shell thick, strong, equivalved, closed, oblong, or suborbicular ; 
ornamented externally with radiating ribs or costa, more or less elevated ; hinge with two 
teeth in the right valve, interlocking, one short and straight, the other oblique and elon- 
gated, with one small lateral tooth ; impression of pedal muscle close to the oral adductor ; 
mantle-mark without a sinus ; connexus ligamental. 
Animal with the edges of the mantle disunited except at the siphonal extremity, 
where they are connected to form two short siphons; branchial margin cirrated ; foot 
elongated. 
The genus, as here employed, is intended to comprise all those transverse or elongated 
forms that were for some time considered to be entitled to generic isolation under the 
name Venericardia, as well as those which are circular or lenticular, possessing the same 
kind of hinge. The line of separation between the transverse and the orbicular is so 
indistinct that it is not possible to define it. Nearly all conchologists are now agreed 
to employ but one generic name, and the above is the older of the two. 
The exterior of the shell has generally radiating and elevated costs, but there are 
some small species with the characteristic form of dentition in which this character is lost, 
the surface not only becoming smooth, but a few have ridges produced by elevated lines 
of growth in a transverse or concentric, and occasionally in an oblique direction. 
These latter species closely approach the genus Asfarte, differmg only in having an 
elongated and oblique tooth in the hinge of each valve. The margins of these small 
species are denticulated ; but, unlike those of Astarte, they appear to be thus ornamented 
at all ages. Those species whose shells are furnished with coarse, elevated, and radiating 
ridges have them produced beyond the margin, where they interlock the one with the 
other; but the small smooth species have the crenulations imposed upon the inner 
margin of the shell. These are quite independent of any external rays, and the animal 
which forms them has probably fimbriated edges to the margins of the mantle. The thick 
and ponderous species not only firmly interlock by the projecting ridges at the margin, 
but the animals are furnished with powerful adductors, deeply implanted in the interior of 
the shell, implymg thereby considerable force for closing the valves. It is said that the 
animal spins a byssus with its foot. All the species I have seen are capable of firmly 
closing the ventral margin of the valves, and appear to be free species. 
In the recent state the shells are covered with an epidermis. The genus has a wide 
geographical extension, but it has more of a tropical or subtropical character than 
otherwise ; one species is living on the coast of Ochotsk, while some inhabit the seas of 
the torrid zone. Its vertical range is also great, extending to 150 fathoms. The animals 
prefer a sandy bottom. In a fossil state it is found as low as the Trias. The species 
