CARDIUM. 265 



which lately hindered our passage and made it so difficult 

 and unpleasant. The present group is free from all 

 those anomalies that rendered the position and arrange- 

 ment of many genera in the last two families extremely 

 critical and indefinite, taxing the discriminative powers 

 of the systematist to an extent which makes concho- 

 logical nature almost faint under the task. The Cockles 

 are easily characterized and cannot be well confounded 

 with any other family. 



Although the mantle-scar is usually entire, Mr. Clark 

 has observed that " in some species an imperfect pallial 

 siphonal sinus may be seen/ 5 I have in vain searched 

 for such a mark. According to Dr. Carpenter the 

 microscopical structure of these shells is very much 

 the same as in the Area family. He says that in various 

 species of Cardium, which he had examined, he found 

 "a considerable amount of tubular structure in the 

 external layer, while in Isocardia there is scarcely a 

 vestige of it thus bearing out the general statement 

 that the presence of the tubular structure has a relation 

 to the costations, foliations, or other sculpturing of the 

 external surface." 



Cockles inhabit sandy and soft ground ; and they are 

 widely distributed over all seas, from low-water mark to 

 considerable depths. 



We have only the typical genus : the others (Car- 

 dissa, Papyridia, and Aphrodita) are tropical and arctic 

 forms. 



Genus CAR'DIUM *, Linne. PL V. f. 9. 



BODY globose : mantle thin at the edges and tubercled be- 

 hind, very slightly adhering to the shell: tubes unequal in 

 size, but nearly of the same length, covered with cirri or fila- 



* Heart-shaped. 



