_Wrinkled Horse Mussel._ * * * * * GENERIC CHARACTER. _Testa transversa, plerumque tenuis. Cardo linearis, edentulus. Lamina cardinalis glabra, aliquando levata, antice sinu sub ligamento desinens. Impressiones musculares 3. Ligamentum externum. Animal fluviatile._ Typus Genericus _Mytilus Anatinus_ Lin. Shell transverse, generally thin. Hinge consisting of a simple marginal lamina without teeth, smooth or slightly raised, terminating at the anterior end in a curve or sinus below the ligament. Muscular impressions three. Ligament external. Animal fluviatile. Generic Type _Duck Mussel_ Pennant. * * * * * SPECIFIC CHARACTER. _A. testa ovato-oblonga, crassa, convexa, antice oblique rotundata; intus fulva, margine rufo; margine cardinali leviter curvato, infra umbones crasso._ Shell ovate-oblong, thick, convex, anterior obliquely rounded; hinge margin slightly curved and thickened beneath the umbones; inside fulvous, with a reddish margin. * * * * * The shells now arranged under the kindred genera of _Unio_ and _Anodon_ are exclusively fluviatile, or inhabitants of fresh water, and are dispersed both in the old and the new world. In the Linnaean system, the first, being furnished with teeth, are placed among the _Myae_; and the latter, from having none, are arranged with the _Mytili_. Two common shells, in our own rivers and ponds (_Mya pictorum_, and _Mytilus anatinus Lin._), will readily present to the student the characters by which they are severally distinguished. The shells of the present genus bear such a general resemblance to each other, and are so simple in their construction, that a corresponding minuteness of discrimination is requisite to characterize the species; I have, therefore, selected for this purpose the modification of the upper margin to which the ligament is attached, and which in other shells forms the bases of the teeth. This I have termed the hinge margin. The form of the notch or sinus which terminates this part will also be found of much importance in discriminating the species; for no shells vary more in their form, thickness, or convexity than these do, according to their locality, age, or other circumstances. Shell transverse, oval; rather thick and ventricose; both extremities obtuse; the anterior side (from the umbones to the exterior margin) obliquely rounded; umbones prominent; hinge margin rather thick, slightly curved, and swelled immediately under the umbones; sinus short, abrupt, curved; epidermis coarse, black, and much wrinkled; inside stained with yellow, and having a narrow reddish rim or margin.