BIVALVES. CARDIUM. 35 



great variety of structure and coloring. There are fifty- 

 t wo species. 



The valves of the Cardium are for the most part of a 

 convex, swollen, or gibbous construction, and often spher- 

 ical; yet, in some instances, their form is elongated and 

 compressed. In other species the contour exhibits the 

 figure of a perfect heart, as in the C. cardissa, &c. 



The shells are usually equivalve, and have their out- 

 sides adorned with longitudinal ridges and grooves, cross- 

 ed by transverse striae, similar to the common cockle, 

 only much more articulate. In others, again, the ridges 

 are beset with rows of acute spines, as in the C. aculea- 

 tum and C. echinatum; but the exteriors of some have 

 a perfectly smooth and polished surface, as the egg-coc- 

 kle, &c. 



The C. fraguni and C. unedo exhibit a peculiar forma- 

 tion, being sub-angular, and only heart-shaped when seen 

 in a particular position, A similar coincidence is observ- 

 able in the C. retusum, though, in other respects, it dif- 

 fers; for the C. fragum and C. unedo have their ridges 

 covered with crowded pink or yellow elevated lunules 

 or crescents, but the C. retusum has nodules rather than 

 imbricated scales. 



The interior margin of this genus is almost universally 

 crenate or toothed. 



The hinge is furnished with two teeth, and a larger 

 remote lateral tooth on each side of it, each locking into 

 its opposite. 



The C. edule, or common cockle, is found in great 

 abundance beneath the surface on sandy coasts; the fish 

 affords a wholesome and nourishing food. 



The most rare and valuable species of the genus Car- 

 dium, is the C. costatum or piperidged cockle, which 



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