56 

 RANGIA. 



GENERIC CHARACTER. 



Shell equivalve, inequilateral ; subcordate ; hinge 

 with an oblique fosset for the cartilage ; right valve 

 with an elongated transversely striated groove to re- 

 ceive the posterior tooth of the opposite valve ; an- 

 terior side with two grooves for corresponding teeth, 

 the smaller one margined by two compressed teeth : 

 left valve with an elongated transversely striated tooth 

 posteriorly, and two anterior ones, the largest being 

 curved, elongated, and angulated posteriorly ; liga- 

 ment short, internal, placed on the superior margin 

 of the fosset ; cartilage divided, an equal portion re- 

 maining in each valve, with a concave surface ; mus- 

 cular impressions two, suboval j palleal impression, 

 with a small sinus. 



OBSERVATIONS. 



This remarkable genus of the family Mactrad^, 

 prefers, like Cyrena, brackish or even fresh water, 

 and has an epidermis much resembling that of Unio. 

 No more than one species has yet been discovered, 

 which some have supposed to be the shell described 

 by Lamarck, as Cyrena truneata, and appears to 

 have been first recognized and described as a distinct 

 genus by Mr. Gray, under the name of Clathradon, 



