Most 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 described so many, the short descriptions which he has given, and the want of figures to elucidate them, render it impossible to determine accurately one half of the species which he has enumerated.
UNIO fragilis.
Fragile River Mussel.
Generic Character.—See Pl. 58.
Specific Character.
- U. testâ transversim ovatâ, tenui, intùs purpurascente; dentibus cardinalibus tuberculatis, sub-obsoletis.
- Shell transversely oval, thin, within purple; cardinal teeth tuberculated, nearly obsolete.
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 development 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.