392 MOLLUSCA. 



ligament placed behind the hinge opens the valves by its con- 

 traction. 



A considerable number of bivalves are provided with what 

 is termed a byssus, or a fasciculus of threads more or less 

 loosely connectedj which issues from the base of the foot, and 

 by which the animal adheres to various bodies. It uses its 

 foot to direct the threads and to agglutinate their extremities; 

 it even reproduces them when cut, but the nature of the pro- 

 duction is not thoroughly ascertained. Reaumur considered 

 these threads as a secretion, spun and drawn from the sulcus of 

 the foot ; Poli thinks they are mere prolongations of tendi- 

 nous fibres. 



The shell essentially consists of two pieces, called valves, to 

 which in certain genera are added others, connected by a 

 hinge that is sometimes simple and sometimes composed of a 

 greater or smaller number of teeth and plates, which are re- 

 ceived into corresponding cavities. 



There is usually a projecting part near the hinge called the 

 suminit or nates. 



Most of these shells fit closely when the animal approxi- 

 mates them, but there are several which exhibit gaping por- 

 tions either before or at the extremities. 



FAMILY I. 



OSTRACEA. 



The mantle is open, without tubes or any particular aper- 

 ture. 



The foot is either wanting in these Mollusca or is small ; 

 they are mostly fixed by the shell or byssus to rocks and other 

 submerged bodies. Those which are free, seldom move ex- 

 cept by acting on the water by suddenly closing their valves. 



In the first subdivision there is nothing but a muscular mass 

 reaching from one valve to the other, as seen by the single 

 impression left upon the shell. 



It is thought proper to class with them certain fossil shells, 



