CLASS I. TROPIOPODA. ORDER II. UNIMUSCULOSA. 129 



PI. XCVI. Fig. 2. 

 Chama Arcinella, Linnaeus, Syst. Nat., p. 1 139. Lamarck, Anim. sans 



vert., new edit., vol. vi. p. 584. Enc. Meth., pi. 197. f. 4, a, b. 

 Arcinella spinosa, Schumacher. 



PI. XCVI. Fig. 3. 



Chama Lazarus, Lamarck, Anim. sans vert., new edit., vol. vi. p. 579. 



Enc. Meth., pi. 196. f. 4, 5. 

 Chama macrophylla, Chemnitz. 

 Chama gryphoides, Bruguiere. 



PI. XCVI. Fig. 4. (fossil.) 



Chama squamosa, Brand, Fossils, vol. vii. p. 86. Sowerby, Genera of 



Shells, No. 7. Enc. Meth., pi. 197. f. 2, a, b, c. 

 Chama lamellosa, Chemnitz. Lamarck. 

 Chama rugosa, Bruguiere. 



Order II. TROPIOPODA UNIMUSCULOSA. 



Testa musculi impressione unica prope ad centrum interne imbuta. 

 Animal interdum bysso affixum. 



Lamarck appears to have been the first to notice the important cha- 

 racter which serves to establish our primary division of the Tropiopoda. 

 Before his time both the bimuscular and unimuscular species of this class 

 were indiscriminately associated together ; indeed in some instances, in 

 one and the same genus. This method was certainly at variance with the 

 true organization of the animal ; and the additional advantage which the 

 present arrangement offers, is that it is founded upon a character which 

 may be determined without referring to the animal ; the points of mus- 

 cular attachment being always more or less distinctly discernible on the 

 internal surface of the shell. Here, however, we meet with the dif- 



vol. i. s 



