342 



THE OCEAN WORLD. 



The twelfth family, Cardiadae, contains the familiar cockles, 

 belonging to the genus Cardium, which is derived from KapSlo, a 

 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 edge's 

 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 groenlandicum, 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. hians (Fig. 143) the mantle has a large opening in front, 

 fringed anteriorly with papillae 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 



