260 MOLLUSCA. 



ascertained. Reaumur considered these threads as a secretion, spun and 

 drawn from the sulcus of the foot ; Poli thinks they are mere prolongations of 

 tendinous fibres. 



The shell consists essentially 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 summit. 



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 bodies. 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 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, the valves ot 

 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 connected by muscles 

 only. They form the genus 



ACARDA. Brugicres, OSTRACITA, La Peyrouse, 



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

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

 the Radiolites, Sptuerulitet, Calceola, Hippurites, Batolithes. 



As to the well-known living testaceous Acephala, Linnaeus had united in 

 the genus 



OSTREA, 



All those which have but a small ligament at the hinge, inserted into a little 

 depression on each side, and without teeth or projecting plates. 



OSSTREA, Brugu&res. 



The true Oysters have the ligament as just described, and irregular, ine- 

 quivalve, and lamellated shells. They adhere to rocks, piles, and even to 

 each other, by their most convex valve. 



The animal, PELOBIS, Poli, is one of the most simple of all the bivalves, 

 possessing nothing remarkable but a double fringe round the mantle, the lobes 

 of which are only united above the head, near the hinge ; but there is no 

 vestige of a foot 



