278 Mr. W. Clark on Ancylus oblongus and A. fluviatilis. ~ XXVI. — On Ancylus oblongus and A. fluviatilis. By William Clark_, Esq. To the Editors of the Annals of Natural History. Gentlemen, Norfolk Crescent, Bath, March 1855. I PRESENT an account of two very interesting members of the subaquatic pulmoniferous family of the Limneadce, the Ancylus oblongus, often termed " A. lacustris " or Velletia lacustris, and the A. fluviatilis, which have heretofore caused some difficulty with respect to their relations with each other and to natural position, and even now excite attention; but I believe these animals have been brought to a pretty safe anchorage by the Eev. M. J. Berkeley. I have in some of our scientific publi-cations read his observations on one or both (?) these species, but they have escaped my memory, and I have not at present the means of reference; the ground therefore is almost new to me, with this advantage, that whatever errors I may commit, will be cor-rected by consulting that eminent naturalist's notes; and perhaps my comparison of the two animals with each other, and with the Limneadce, may offer some new points of view, both as regards their internal anatomy and external aspects, and the generic considerations connected with natural position. I have been tempted to enter on this memoir in consequence of the extra-ordinary abundance, at Exmouth in 1854, of both these species, and one of the standards of comparison, the Limneus pei^eger. No genus has received greater changes of position than An-cylus ; the only two British species have even been consigned to separate genera, the Velletia lacustris and A. fluviatilis, — a most unfortunate disseverance, as the organs of both are all but iden-tical. The animals have been pronounced at one time Pectini-branchiates, at another Cervicobranchiates, and agreeably to the surmises of conchologists have been passed to and fro, from their most ancient site as Cyclobranchiates, to Haliotis, Crepidula, &c. The zoologists who have assigned them a natural location are Mr. Berkeley, Dr. Gray, and M. de Ferussac : as to the Rev. L. Guilding, whom M. Deshayes quotes as a dominant authority, his account of the animal of Ancylus is very incorrect. As I consider Ancylus more in harmony with the Limneadan type L. pereger than either Physa and Planorhis, I should have preferred depositing it as a section of that genus, but I fear the present race of malacologists are not prepared for so decided a measure. I have adopted the A. oblongus, the Velletia, nonnull., from having given it a close examination, and as being the dextrorsal species, for the type of the genus Ancylus, and the
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