ACEPHALA TESTACEA. 
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. 
FAMII.Y 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 suddenly closing their valves. 
In the first subdivision there is nothing but a muscular mass reacli- 
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 wei’e con- 
nected by muscles only. They form the genus 
Acarda, Brug. — Ostracita, La Peyr., 
Of which M. de Lamarck makes a family that he names Rudista. 
The shells ai’e 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 Discinje, Lam., are Orbiculse ; it is also thought that liis 
Craniae should be approximated to them. The Jodamies of M. de France or 
Birostrites, Lam., are mere moulds of Sphcerulites or at least of the bodies 
always found in their interior, although they do not adapt themselves to their form. 
See M. Charles Desmoulins on the SpheruHfes. 
