# Zoological Illustrations/VolII-Pl96

ANODON rugosus,
Wrinkled Horse Mussel.
Generic Character.
    Testa transversa, plerumque tenuis. Cardo linearis, edentulus. Lamina cardinalis glabra, aliquando levata, anticè 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. 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; 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.
For this species, now, I believe, first made known, I am indebted to G. C. Bainbridge, Esq. of Liverpool, who received several specimens from the United States. It appears to have been unknown to Mr. Say, who has published an account of the land and river shells of North America.
The student might be led to suppose, that the two genera of Unio and Anodon are strongly characterized; for the first includes many of the most ponderous bivalves yet discovered, and the second some remarkably thin and brittle. Among the Uniones are shells furnished with hinges of the greatest force, while most of the Anodons are perfectly destitute of any; nevertheless, the gradations by which these characters approach each other are very remarkable, and some shells which partake of both have been arranged in separate genera. Of these, the best defined are Hyria Lam. and Dipsas of Leach; the one allied nearest to Unio, but having the cardinal teeth assuming the appearance of lateral or lamellar teeth; the other more resembling the Anodons, but furnished with a strongly defined and elevated lamellar tooth, extending the whole length of the hinge. Between these two genera should be placed another of Lamarck's, called by him Iridinia, which has likewise only a simple lamellar plate, but broken into a great number of crenated teeth. The observing Mr. Say has likewise proposed another under the name of Alasmodonta, which, however, I shall take another opportunity of noticing.
I have ventured to exchange the ungrammatical name of Anodonta, given by Bruguiere to this genus, for Anodon, at the suggestion of the learned Dr. Goodall, Provost of Eton College.