BIVALVES CARDIUM. 19 



and compressed. In other species, however, the contour 

 exhibits the figure of a perfect heart, as in the Cardiuni 

 Cardissa, &c. &c. 



The siiells are usually equivalve, and have their out- 

 sides adorned with longitudinal ribs and grooves, crossed 

 by transverse striae, something in the manner of the com- 

 mon Cockle, only much more articulate. 



In others, again, the ribs are beset with rows of acute 

 spines, as in the Cardium Aculeatum and Echinatum; 

 but the exteriors of some, on the contrary, exhibit a per- 

 fectly smooth and polished surface, as in the Egg -Cockle, 

 &c. &c. &c. 



The Cardium Fragum, and Unedo, exhibit a peculiar 

 formation, for they are sub-angular, and are only heart- 

 shaped when seen in one particular position. A similar 

 coincidence is observable in the Cardium Retusum, 

 though, in other respects, it differs ; for the Fragum and 

 Unedo have their ribs covered with crowded pink or yel- 

 low elevated lunules or crescents, whereas, the Retusum 

 has nodules rather than imbricate 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 one on each side of it, each locking into its 

 opposite. 



The Cardium Edule, Or common Cockle, is found in 

 great abundance beneath the sand on sandy coasts ; the 

 fish afford a wholesome and nourishing food. 



The most rare and valuable species of the genus Car- 

 dium, is the Costatum or pipe-ridged Cockle, which 

 has rows of white hollow elevated ribs, situated at regu- 

 lar distances on its surface, and proceeding in a longitu- 

 dinal direction from the beaks to the margin ; the inter- 



