66 ' CONCHOLOGY. 



covered with epidermis, and flexed anteriorly ; hinge with two 

 lamellar teeth transversely striated, one distant, posterior and 

 shortened, the other anterior, long, and lateral ; ligament ex- 

 terior. Habitation unknown. One species. 

 Castalia ambigua. 



FAMILY XIII. 



Naiadea. Four genera. 



1. Genus Unio. PI. VIII. 



Animal, See Castalia. 



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

 dermis, corroded at the summits, which are dorsal and suban- 

 terior ; 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 impressions, well marked, besides those of the retractila 

 muscles. The species of this genus grow more numerous daily ; 

 they are found in all countries, but particularly in North Ame- 

 rica. One hundred and sixty-seven defined species. Numerous 

 fossil. 



Unio Batavus. Unio crassidens. 



U. ciassissimus. U. obHquus. 



U. elongatus. U. plicatus. 



U. litoralis. U. purpuratus. 



U. plctorum. U. radiatus. 



U. platyrhonchus. U. rectus. 



U. marginalis. U. retusus. 



U. tigris. U. rotundatus. 



U. Egyptiacus. U. Australis. 



U. Niloticus. U, Nova Hollandica. 



U. Leaii. 



The following species are American : 



Unio Bengalensis. Lea. U. lamellatus. Lea. 



U. bilineatus. „ U. Morchisonianus, „ 



