148 BIVALVES. 



change the position of their shells ; sometimes 

 it is transformed into a paw, and sometimes 

 it is employed to fix the silky filaments of a 

 byssus. These mollusks do not appear to possess 

 the different organs of sense, but to be reduced 

 to that of touch ; indeed their faculties are 

 altogether much less developed than those of the 

 mollusca cephale. None of the bivalves are 

 terrestrial shells, some few are fluviatile. The 

 generic character of the bivalves is principally 

 derived from the formation of the hinge, and the 

 general appearance of the shell. 



PARTS OF A BIVALVE SHELL. 



The valves. 



The cardo or hinge, the part where the valves 

 are united. 



The beaks or apices, the points of the valves 

 near the hinge. 



The base, the part of the shell opposite the 

 beaks. 



The umbones or bosses, the swelled parts near 

 the beaks. 



The ligament, the elastic horny substance, 

 which connects the two valves. 



The margin of valves. 



The area or anterior slope, the margin of the 

 valves near the ligament. 



The areola or posterior slope, the margin of 

 the valves, the other side of the ligament. 



