ACEPHALA TESTACEA. 273 



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 impress- 

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

 the Radiolites, Sphserulites, Calceola, Hippurites, JBatolithes. 



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. 



OSTREA, Brug. 



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

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

 each other, by their most convex valve. 



The animal, PELORIS, 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. 



PECTEIT, Brug. 



The Pectens, very properly separated from the Oysters by Brugieres, al- 

 though they have the same kind of hinge, are easily distinguished by their 

 inequivalve semi-circular shell, almost always regularly marked with ribs 

 which radiate from the summit of each valve to the edge, and furnished 

 with two angular productions called ears, which widen the sides of the hinge. 



LIMA, Brug. 



The Limss differ from the Pectens in the superior length of their shell in 

 a direction perpendicular to the hinge, the ears of which are shorter, and 

 the sides less unequal, thus forming an oblique oval. The ribs of most of 

 2 K 



