64 



terior slope, and profoundly emarginate posteriorly, 

 as represented in the outline, fig. 3. 



UNIO OCCIDENTALS. 



Plate XXXVI. Fig. 1. 



DESCRIPTION. 



Shell elliptical, convex, anterior margin very regu- 

 larly rounded; anterior dorsal margin elevated; basal 

 margin straight and parallel with the ligament mar- 

 gin; posterior margin oblique, extremity rounded; 

 umbonial slope undefined; beaks slightly prominent, 

 with undulated grooves; summit rounded; epidermis 

 yellowish, with narrow green rays, which are obso- 

 lete or wanting on the anterior side; within bluish; 

 cardinal teeth direct, prominent, acute; lateral teeth 

 lamelliform. 



OBSERVATIONS. 



This shell has an outline similar to that of U. com- 

 planatus. It differs from that species in its yellow 

 epidermis, the peculiarity of its rays, more elevated 

 anterior margin, lamelliform lateral teeth, &c. 



Inhabits Currant river, Arkansas, whence it was 

 brought by Mr. Featherstonhaugh, to whose kindness 

 I owe the use of this and several rare and beautiful 

 species. 



