In prosecuting my illustrations of this genus, I have carefully examined all the specimens in the cabinets of my friends, and have added many to my own. These materials have thrown some additional light on those species which I have already described, and have enabled me to detect several others altogether new. Among the latter is the shell here figured, and which is so rare, that I know but one example of it in this country. Its form is more oval than that of _A. rugosa_, from which it is likewise distinguished by a very small umbilicus, nearly concealed by the inner lip; the wrinkles are numerous and unequal, the spire pointed, and the aperture milk-white. Since my remarks on the _Planorbis cornu-arietis_ of Lamarck were published, it has been discovered that the shell is furnished with an operculum: one of these is in the possession of Mr. Sowerby: thus what was a matter of doubt becomes a fact, and affords the only substantial argument for terming it an _Ampullaria_. On the other hand, its affinities to _Planorbis_ (marked by its discoid, depressed form, and the total absence of the pillar,) remain in no degree impaired. The weight of argument on both sides _now_ appears to be so equal, that it is a matter of no moment whether this shell be placed in the system at the end of the _Ampullariae_, or at the commencement of the _Planorbes_. To the generality of conchologists, the latter collocation would appear the most simple; but, on the whole, I incline more to the propriety of considering it the terminal species of the _Ampullariae_, or that which marks their transition (as I before observed) to the _Planorbes_. * * * * * Pl. 176 [Illustration] ANODON elongatus, _Lengthened Anodon._ * * * * * GENERIC CHARACTER.--See Pl. 96. * * * * * SPECIFIC CHARACTER. _A. testa transversim oblonga, crassa, antice compressa, extremitate utraque rotundata; umbonibus valde prominentibus, crassis; lamina cardinali convexa._ Shell transversely oblong, thick, anteriorly compressed, both extremities rounded; umbones very prominent, thick; hinge-plate convex. * * * * * This extremely rare shell bears not the least resemblance to any which Lamarck has described, or with which I am acquainted. It was formerly in the late Mr. Forster's collection, and is now in the possession of Mrs. Mawe. Its form is like that of _Unio ovatus_ (_Mya ovata_ of Montague), but it is a much thicker and stronger shell; the posterior end is greatly compressed, but round; the umbones convex, remarkably thick, and deeply eroded; the inside pearly and iridescent, with a strong flesh-coloured tinge; the ligamental or hinge-plate is perfectly smooth, and rather convex; the muscular impressions are deep.