268 DIMYARIA.—CARDIADA, 
FAMILY CARDIADZ. 
(Cockles.) 
This is a large, important, and, generally speak- 
ing, well-defined group of bivalves, the characters 
of which are thus given by Cuvier. In all the 
mantle is open anteriorly, but posteriorly it pre- 
sents two siphons or orifices, one for respiration, 
the other for the egesta. ‘These form tubes some- 
times distinct, sometimes united into a single mass. 
There is always a transverse adductor muscle at 
each extremity of the shell, and a foot which gene- 
rally serves for the purpose of locomotion. It may 
be laid down as a rule, that the species with elon- 
gated siphons live buried in the mud, or sand. 
This condition of organization may be recognised 
on the shell, by the more or less developed contour 
which the impression of attachment of the borders 
of the mantle describes, before uniting with the 
impression of the posterior transverse muscle; ”’ 
or, in other words, by the course of the pallial 
impression. 
These Mollusks are all marine, with one or two 
doubtful exceptions; they frequently attain large 
size, and are valued for the delicacy and wholesome- 
ness of their flesh; their abundance often renders 
them easy to be procured. 
GENUS CARDIUM. 
The shells of this well-known genus are globose 
and heart-shaped: the valves are equal, with beaks 
rolled in towards each other. The hinge is formed 
by four teeth in each valve, of which the two pri- 
