CONCHOLOGY. 



FAMILY XIII. 



Naiadea. Four genera. 



1. Genus Unio. 



Anivial. See Castalia. 



Shell. Usually very thick, nacred within, covered with epider- 

 mis, corroded at the summits, which are dorsal and subanterior ; 

 dorsal hinge formed by a double precardinal tooth, more or less 

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

 the right, together with a long lamellous tooth under the ligament ; 

 ligament external, dorsal and postapicial ; two muscular impres- 

 sions, well marked, besides those of the retractile muscles. The 

 species of this genus grow more numerous daily ; they are found 

 in all countries, but particularly in North America. One hundred 

 and sixty-seven defined species. Numerous fossil. 



Unio Batavus. 

 U. crassissimus. 

 U. elongatus. 

 U. litoralis. 

 U. pictorum. 

 U. platyrhnchus. 

 U. marginalis. 

 U. Leaii. 

 U. tigris. 

 U. Egyptiacus. 

 U. Niloticus. 



Unio crassidens. 



U. obliquus. 



U. plicatus. 



U. purpuratus. 



U. radiatus. 



U. rectus. 



U. retusus. 



U. rotundatus. 



U. Australis. 



U. Nova Hollandica. 



The following species are American. 



Unio Bengalensis. 

 U. bilineatus. 

 U. cseruleus. 

 U. corrugatus. 

 U. Corrianus. 

 U, Graianus. 

 U. alatus. 

 U. andontoides. 



Lea. 



U. lamellatus. 



U. Morchisonianus. 



U. olivarus. 



U. ponderosus. 



U. divaricatus. 



U. acutissimus, 



U. fabalis. 



U. Fisherianus. 



Lea. 



