ACEPHALA TKSTACEA. 83 



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 received into corresponding 

 cavities. 



There is usually a projecting part near the hinge called the sum- 

 mit or nates. 



Most of these shells fit closely when the animal approximates them, 

 but there are several which exhibit gaping portions either before or 

 at the extremities. 



FAMILY I. 



OSTRACEA. 



The mantle is open, without tubes or any particular aperture. 



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 bo- 

 dies. Those which are free, seldom move except by acting on the 

 water by sviddenly closing their valves. 



In the first subdivision there is nothing but a muscular mass reach- 

 ing 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, the valves 

 of which do not even appear to have been held together by a ligament, 

 but which covered each other like a vase and its cover, and were con- 

 nected by muscles only. They form the genus 



x^CARDA, Brug. — OsTRACiTA, Lct Peyr., 



Of which M. de Lamarck makes a family that he names Rudista. 

 The shells are thick, and of a solid or porous tissue. They are now 

 divided into the 



Radiolites, Lam., 



In which the valves are striated from the centre to the circumfe- 

 rence. The one is flat, the other thick, nearly conical and fixed*. 



* The species of Brugi^re, 173, f. 1, 23, which forms the genus Acarda, Lam., 

 appears to be nothing more than a double epiphysis of the vertebra of some ceta- 

 ceous animal. The Discin.e, Lam., are Orbiculse ; it is also thought that his 

 Cranise should be approximated to them. The Jodamies of M. de France or 

 BiROSTRiTES, Lam., are mere moulds of Sphceruhtes or at least of the bodies 

 always found in their interior, although they do not adapt themselves to their form. 

 SeeM. Charles Desmoulins on the Sphentlifes. 



G 2 



