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 



