46 Lamarck's Genera of Shells. 



flat valve, when the muscle is contracted. The perforated flat 

 valve is usually considered as the lower one in this genus, as 

 being that which tests on the bodies to which the shell is at- 

 tached; whilst with the ostrese, the larger and most concave is 

 correctly styled the lower valve. The contrary is the case with 

 the terebratulae, because it is the largest and most concave 

 valve of that shell which is perforated at the beak. Indepen- 

 dently of the muscular attachment of the animal to the oper- 

 culum, the two valves are connected by an internal, cardinal 

 ligament, the impression of which is very perceptible. 



The organization of the animal, according to Poll, is similar 

 to that of the oyster. 



Type. Anomia ephippium *. (Idem, Linn.) 



Shell suborbicular, rugose-plicate, wavy, rather flat ; foramen 

 oval. 

 , Mediterranean. LaMaticha,&c. 9 Species. PI. II. Fig. 97. 



Section 3rd. 



Ligament either none, or unknown ; or represented by a ten- 

 dinous cord which supports the shell. 



The shells of the two preceding sections have true, known 

 ligaments ; those of the one we are now entering on have in 

 reality no true ligament, for the tendinous cord observed in 

 some of them, is merely the extremity of the muscle of attach- 

 ment of the animal, which passes through a hole, in the large 

 beak of the shell, and fixes itself to foreign substances, but by 

 no means serves to support the valves. This section contains 

 two families. 



1st. Family. 

 RuDiSTA. (6 Genera.) 



Ligament, hinge , and animal uuknov.n. Shell very incqui- 

 valve. No distinct beaks. 



The two remaining families, the last of the conchifera, pre- 

 sent us with very singular shell-fish, sometimes in consequence 

 of the form of the shell, and sometimes from the peculiarities of 

 the animal, of which we find no example in the other conchifera. 

 ♦ Saddle. 



