CHAP. IX. 



BIVALVES: 

 CARDIUM ; Cockle, or Heart Shell. 



Q. WHAT have you to tell me respecting the 

 Cardium ? 



A. That the shell is nearly equilateral and 

 equivalve, generally convex, longitudinally ribbed, 

 channelled, margin-toothed; that there are two 

 centre teeth under the beak in each valve, stand- 

 ing in contrary positions : in one valve side by 

 side, in the other valve one before the other. 



Q. Are these the only characteristics ? 



A. No. One obvious character of the Car- 

 dium is, that the shell is generally strong and 

 thick, very convex, and mostly with strong, pro- 

 minent, longitudinal ribs on the surface. The 

 disk is usually convex, but sometimes much 

 depressed, like a keel, or angular. The ribs and 

 furrows of the two valves are so disposed as to 

 alternate at the margin, and to lock acutely and 

 firmly into each other. 



