36 GENERA OF SHELLS. 



NAIADA. 



Fresh water shells, the hinge of which is sometimes furnished 

 with an irregular., simple, or divided cardinal tooth, and a 

 longitudinal one, which extends under the corselet ; some- 

 times no tooth, or furnished with irregular, granular tuber- 

 cles throughout ; muscular impression posterior, compound; 

 beaks decorticated, often eroded. 



The naiada are well distinguished from the fresh 

 water conchse by their hinge and inhabitant. The 

 shell is free, regular, equivalve, inequilateral, always 

 transverse ; the epidermis is greenish, inclining to 

 brown, and always wanting at the beak. The mus- 

 cular impressions are. lateral and quite separate ; that 

 of the posterior side is composed of two or three dis- 

 tinct, unequal impressions, which distinguishes them 

 from the other bimuscular conchifera. 



UNIO. Shell transverse, equivalve, inequi- 

 lateral, free ; beaks decorticated, suberoded ; 

 muscular impression posterior, compound ; 

 hinge with two teeth on each valve ; one car- 

 dinal, short, irregular, simple or bipartite, sub- 

 striated, the other elongated, compressed, late- 

 ral, prolonged under the corselet ; ligament 

 external. 



The unio is very distinct from the anodonta, which 

 it resembles in shape, by its hinge. Each valve has a 

 short cardinal tooth, that on the left generally simple, 

 that on the right divided into two lobes, besides a late- 

 ral tooth. The two teeth articulate together when the 

 valves are shut. The shell is generally formed of a 

 very brilliant mother of pearl. The lamina of the 

 margin of the shell, above the lateral tooth, has a trun- 



