66 CONCHOLOGY. 



FAMILY XIII. 



Naiades. Four genera. 



1. Unio. Forty-eight species. 



The species of this genus beconae every day more nu- 

 merous ; they are found in all countries, but particularly in 

 North America. The Unio is a fresh-water shell, and there- 

 fore, with great propriety, removed from the Mya, which con- 

 sists entirely of marine shells. The substance is pearly; 

 the exterior covered with a brown or green epidermis ; the 

 apices eroded. They are found in the mud of rivers, with 

 their apices downward ; some are slightly gaping, and some 

 species produce fine pearls. 



Shell generally very thick, pearly within, covered with 

 epidermis ; summits eroded, dorsal, and sub-interior ; be- 

 sides a long lamellous tooth under the ligament, the hinge 

 is formed by a double precardinal tooth, more or less com- 

 pressed, and dentated irregularly on the left valve, simple on 

 the right valve ; ligament external, dorsal, and post-apicial ; 

 muscular impressions well marked. 



Unio sinuata. Unio carinifera. 



U. elongata. U. Georgina. 



U. crassidens. U. clava. 



U. Peruviana. U. recta. 



U. rariplicata. U. naviformis. 



U. pupurata. U. glabrata. 



U. ligamentina. U. nasuta. 



U. obliqua, U. ovata. 



U. retusa. U. rotundata. 



U. rarisulcata. U. littoralis. 



U. coarctata. U. semirugata. 



U. purpurascens. U. nana. 



U. radiata. U. alata. 



U. brevialis. TJ. deladonta. 



U. rhombula. U. sulcidens. 



