155 



which either shut into each other or into corresponding 

 grooves in the opposite valve. Those teeth which are most 

 decidedly beneath the beakS; in the middle, are termed the 

 cardinal or hinge teeth, and the distant ones the latei^al or 

 accessory teeth. In some shells there are no decided teeth ; 

 the motion being regulated by the ligament being attached 

 to a testaceous protuberance, termed a callus. The teeth 

 themselves vary also in their forms, situations, &c. as will 

 be seen in the description of the several genera. 



A knowledge of the action of the ligament, which, it has 

 been mentioned, serves to attach the valves, is necessary 

 to be possessed by those who enter into the investigations 

 frequently demanded respecting fossil bivalves. This li- 

 gament is sometimes placed externally, and sometimes in- 

 ternally. When external, it necessarily becomes stretched 

 when the shell is closed ; and then, if the muscle which 

 holds the valves together becomes relaxed, it opens them by 

 its elasticity only : but, if internal, between the valves, it 

 becomes compressed when the valves are closed, and then 

 opens them by its elasticity, as soon as the muscular action 

 diminishes or ceases. 



Multivalve shells diifer materially in their form and 

 structure : some, as the pholas, may be considered, from 

 their having their two sides of the same form and dimen- 

 sions, as equivalved ; others, as the anomia, are inequivalved. 

 Some have their valves joined by a squamose kind of suture, 

 as the balanus ; others have their valves united in a ten- 

 dinous peduncle ; whilst, in others, the valves are contained 

 in a testaceous tube. 



The illustrious Linnaeus disposed all the shells which 

 were known in his time under thirty-six genera, founding 

 their generic distinctions, in the univalves, chiefly on the 

 characters of their openings, and, in the bivalves, on those 

 of their hinge. But these genera were found insufficient 

 for the necessary distinctions : shells essentially different 



