342 THE OCEAN WORLD. 
The twelfth family, Cardiade, contains the familiar cockles, 
belonging to the genus Cardium, which is derived from kapdle, @ 
heart, which they are supposed to resemble in form: they are amongst 
the most widely-distributed of shells. The shell is convex, as we see 
in C. hians (Fig. 143), somewhat heart-shaped, equivalved, the edges 
dentate or corrugated, the hinge furnished with four teeth upon each 
valve. The accessary ornamentation varies with the species, some 
being smooth, as in Cardium grocniandicum, Chemnitz (Fig. 144) ; 
others, and by far the greater number, are furnished with regular ripples, 
Fig. 143.—Cardium hians (Brocchi). Fig. 144.—Cardium groenlandicum (Chemnitz) 
generally obtuse, sometimes in ridges diverging from. the point and 
armed with straight or curved spines, arranged in the oddest manner, 
as in Cardium aculeatum (Fig. 145). 
In the genus Cardium, as well as Donax, Tellina, and Venus, the 
respiratory organs are somewhat modified, so as to adapt them to the 
habits of the animal. All these molluscs live buried in the sand, and 
the two siphonal tubes, issuing from the interior of their bodies to 
bring the atmospheric air into communication with their respiratory 
organs, are usually very short. 
In C. Atans (Fig. 143) the mantle has a large opening in front, 
fringed anteriorly with papille in the form of tentacula; the in- 
habitant of the shell has a very large foot; its mouth is transverse and 
funnel-shaped, and furnished with labial appendages. One of the 
