Zoological Illustrations/VolI-Pl57



nasutus.

Rostrated River-Mussel.


 * Testa transversa, æquivalvis, non affixa; natibus decorticatis, suberosis; impressio muscularis postica composita. Dens cardinalis unicus, brevis, irregularis, simplex aut bipartitus, substriatus; dentes laterales duo, elongati, compressi, infra pubem producti.

Typus Genericus Mya Pictorum. Linn.


 * Shell transverse, equivalve, not affixed, the tops decorticated. Posterior muscular depressions double. Cardinal tooth one, short, irregular, simple or double, striated; lateral teeth two, elongated, compressed, and prolonged beneath the corslet.

Generic Type Mya Pictorum. Linn.


 * U. (Div. 2.) testâ transversim elongatâ, margine dorsi rectâ, anticè angulatâ obliquè attenuatâ, extremitate subtruncatâ.


 * Unio (Div. 2.). Shell transversely elongated: dorsal margin straight; anterior side angulated, obliquely attenuated, the extremity slightly truncated.


 * Lister, tab. 151. fig. 6.


 * Unio nasutus. Say in Encycl. Am. Conch. pl. iv. fig. 1.

This is one of the most natural genera in the modern systems of conchology, as it includes all fresh-water bivalves having two rough cardinal teeth in one valve and one in the other. The colours of all are more or less dark-brown, sometimes radiated with green; but the specific characters rest on the contour of the shell and the proportion of the teeth.

There can be no doubt this shell is the Unio nasutus of Say, who refers to the figure of Lister. The Unio nasuta however of Lamarck I apprehend will be found different, as he seems to think; his shell also is purple inside with short thick teeth; indeed so much uncertainty hangs on the shells of this genus, that the species can only be fixed by ample descriptions and very correct figures. The figure is from a shell in the Linnæan Society's cabinet. Mr. Say says it is common in the Delaware River, North America.