BRITISH MOLLUSCA. 



CARDIAD^. 



The Cockles, properly so called, form a very natural 

 assemblage, presenting well-marked characters both of shell 

 and animal. Their shells are of variable thickness, some- 

 times strong, in other cases very tender, and are often sculp- 

 tured in bold and striking relief. The furrows upon them 

 are usually radiating, a feature which prominently dis- 

 tinguishes them from the majority of shells in the last 

 family, and the ridges are often ornamented by arched 

 scales. The dentition of the hinge is very variable, often 

 strongly marked, sometimes nearly obsolete. The pallial 

 sinus is entire, corresponding to the structure of the mantle 

 and siphons, the latter being distinct, but very short, and 

 nearly sessile. Their bases and sides are fringed. The 

 mantle is freely open in front. The foot is remarkable for 

 its geniculated form and great size, enabling the animal to 

 make leaps prodigious in proportion to its dimensions. 



Isocardia is often included by authors in this family; 

 but, as we have already seen, it has much nearer affinities 

 with Cyprina and its allies. The only British genus truly 

 a member of the group is Cardium ,■ one which, from the 

 number, abundance, and beauty of its species, plays no un- 

 important part in our marine fauna. The family connects 

 the tribe last treated of with the Kelliada and Cycladidce, 



VOL. II. B 



