62 CLASS I. TROPIOPODA. ORDER I. BIMUSCULOSA. 



GNATHODON, Gray. 



Testa subovalis, sequivalvis, insequilateralis ; epidermide olivacea induta, 

 ad umbones plerumque paululum decorticata ; dentibus alterius 

 valvse lateralibus duobus cum cardinali unico, alterius lateralibus et 

 cardinalibus duobus, ligamento in foveola interjacente. Tmpressio 

 muscularis pallii sinu parvo. 



The genus Gnathodon was created by Gray for the purpose of intro- 

 ducing a new variety of Mactracea, with a shell resembling, in general 

 appearance, that of some of the fluviatile Conchacea ; the position of the 

 ligament, however, sufficiently indicates the family to which it belongs. 



The shell of Gnathodon is nearly oval, equivalve, inequilateral, and 

 covered with an olive-green epidermis, generally a little decorticated at 

 the umbones ; it has a lateral and two cardinal teeth in one valve, shut- 

 ting between two cardinal and two lateral teeth in the other. The liga- 

 ment is internal, inserted in a deep pit situated between the cardinal and 

 posterior lateral teeth, and the impression of the mantle exhibits a small 

 sinus. 



Example. 



PI. XLIII. Fig. 1 to 4. 



Gnathodon cuneatus, Gray. American Journ. of Science. Sowerby, 

 Genera of Shells, No. 36. 



CRASSATELLA, Lamarck. 



Testa aequivalvis, transversa, insequilateralis, undique clausa ; epider- 

 mide fusca induta. Cardo in valva altera dentibus duobus ; in 

 altera dente unico ; cum foveola in utraque valva antice adjecta, 



