by William Swainson

“ 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 ”

“ A. testâ ovato-oblongâ, crassâ, convexâ, anticè obliquè rotundatâ; intus fulvâ, 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 Linnæan system, the first, being furnished with teeth, are placed among the Myæ ”