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. * * * * * Pl. 97 [Illustration] MARGINELLA, _Date Shell._ * * * * * GENERIC CHARACTER. _Testa ovata. Spira brevissima aut nulla. Labium exterius crassissimum, margine interiore crenato. Columella plicata. Basis subintegra. Animal capitatum, capitis fronte profunde emarginato; oculis ad tentaculorum 2 subulatorum basin externam adsitis; tuba jugulari simplici; pede magno, foliaceo, pone attenuato; penula dilatata, testae latera obtegente._ Typ. Gen. _Volutae perspicula, glabella, prunum, &c._ Lin. Shell oval. Spire very short or concealed. Exterior lip very thick, with the internal margin crenated. Pillar plaited. Base nearly entire. Animal capitate; head notched in front, with lengthened, pointed tentacula, at the external base of which are the eyes; neck with a simple tube; foot large, foliaceous, pointed behind; mantle dilated, and folded over the sides of the shell. Generic Types _Vol. perspicula, glabella, prunum, &c._ Lin. * * * * *