The beautiful shells here selected as a further illustration of the Citron Bulimus not only show the great variability of the species, but clearly prove that _B. virescens_ is, as I suspected, only a variety of _B. citrinus_. In the shell at fig. 1. the upper part of the inner lip (like that described in the Bligh Appendix), is entirely wanting; although it bears, in every other respect, the appearance of a full-grown shell; the umbilicus likewise is open; but in the shell at fig. 2. and 3. the inner lip is quite perfect, and consequently folds over the umbilicus; thus the connexion between the green and yellow varieties is completely established. I have subjoined a slight arrangement of the principal varieties; and have only further to observe, that the specific character I first proposed, appears to me the only one by which this species may be truly distinguished. Since the description of _B. aureus_ at pl. 47 was written, I have seen several other specimens; all of which tend to confirm my belief that it is distinct from _B. citrinus_. * * * * * Pl. 167 [Illustration] ANODON crassus, _Thick Anodon, or Horse Mussel._ * * * * * GENERIC CHARACTER.--See Pl. 96. * * * * * SPECIFIC CHARACTER. _A. testa ovata, lata, crassa; margine cardinali subarcuato, extremitate utraque angulata alata; umbonibus prominentibus, apicibus retusis._ Shell oval, broad, thick; hinge margin subarcuated, the extremities winged and angulated; umbones prominent, the tips retuse. * * * * * It is only recently that travellers have directed their attention to the less attractive shells of distant regions; and our cabinets now begin to be enriched by the numerous land and fresh-water species of those countries. Among these new acquisitions, the fresh-water bivalves appear the most extraordinary in their formation, and the most numerous in species. Of the Anodons, it may be doubted whether the great Linnaeus was acquainted with more than three or four species; Lamarck enumerates fifteen, but a much greater number have passed under my own examination. The species now illustrated is very peculiar; it is a strong, thick shell; in form resembling _Hyria corrugata_, Lamarck; having both extremities winged and compressed; the umbonial slope[6] elevated, and somewhat angulated; the umbones thick and prominent, but obtuse, or nearly truncate, at their apex; the outside of the shell of a dark grass-green colour, and nearly smooth, excepting at the posterior side, which is marked by sulcated striae following the lines of growth; the inside is opaque and whitish, with rich iridescent margins: the muscular impressions deep, and the hinge margin quite smooth.