148 BIVALVES. 



position of their shells ; sometimes it is transform- 

 ed 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 fluvi- 

 atile. 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. 



