UN 10 fragilis. 

 Fragile River Mussel. 



Generic Character. — See PI. 58. 



Specific Character. 



U. testd transxersim oiatd, tenui, intas purpurascente ; dentibus 

 cardinalibus tuberculatis, sub-obsuletis. 



Shell transversely oval, thin, within purple ; cardinal teeth tubercu- 

 lated, nearly obsolete. 



jyiosT fresh water bivalves are remarkably destitute of that 

 variety of colouring, which diversifies the exterior of marine 

 shells, and renders their distinction comparatively easy. A 

 uniform olive green, or brown tint, pervades all the fluviatile 

 genera ; their specific distinctions rest on characters which 

 frequently require long and perplexing descriptions, but 

 which can be explained by the artist with ease and precision. 

 It follows, therefore, that accurate figures of these shells are 

 more particularly wanted ; for, although Lamarck has de- 

 scribed so many, the short descriptions which he has given, 

 and the want of figures to elucidate them, render it impos- 

 sible to determine accurately one half of the species which 

 he has enumerated. 



Unio fragilis is principally distinguished by the cardinal 

 teeth : those in the right valves are 2 ; short, obtuse, and 

 nearly obsolete, more resembling tubercles, than the crenated 

 or striated teeth of this genus. The left valve has one tooth 

 rather sharper. In young shells the ligamental margin is 

 nearly straight, and its extremity somewhat angulated ; but old 

 shells lose these appearances, and become perfectly oval. 

 In both stages of growth the shell is very thin, convex, and 

 the inside (near the umbones) purple. The slight develop- 

 ment of the cardinal teeth brings this shell nearer to the 

 genus Anodon, than any other Unio which I have seen. 



It inhabits the rivers of North America, and was sent to 

 me by Professor Rafinesque. 

 PI. in. 



