186 LIMN^AD^. 



icater limpets, as they are popularly designated, to marine 

 Patellw, for a long time misled naturalists as to the true 

 nature and position of the animal of Ancylus. This dis- 

 cussion has, however, been set completely at rest ; the 

 affinities of the genus are now understood, and its place is 

 universally recognized as being in immediate proximity 

 with Limnaus, P/iT/sa, and Planorhis. Our two indigenous 

 fresh-water limpets diflfer from each other in some re- 

 markable particulars, and are regarded by many naturalists 

 as types of distinct genera, under the names of Ancylus 

 and Velletia. Until we shall know more of the animals 

 of exotic forms, we are disinclined to accept so great a dis- 

 tinction, believing, however, that it may hold good even- 

 tually. In favour of such a view we may quote the ob- 

 servations on the differences in their dentition, as described 

 by Mr. W. Thomson : — "In Ancylus there are thirty simi- 

 lar lateral teeth in a straight line on each side of the 

 central tooth, and then there is a slight curve through a 

 series of six more teeth, where a trifling change in their 

 form occurs. In Velletia, on the contrary, no part of the 

 horizontal row is straight ; its central part is much arched, 

 and is composed of the central tooth and twelve lateral 

 teeth on each side, which do not alter much in form. 

 Then comes one tooth of a different form, and lastly six 

 more on each side, which latter are in a slight curve." 



A. FLuviATiLis, Muller. 



Conoid : aperture roundisli oval ; apex directed to the right. 

 Plate CXXII. fig. 4. 



Patella lacustris, LiST^. Syst. Nat. ed. 12, p. 1260. — Penn. Brit. Zool. cd. 4, 

 vol. iv. p, 143. — PrLTENEY, Hutchins, Ilist. Dorset, p. 51. — 

 DoNOV. Brit. Shells, vol. v. pi. 147. — Maton and Rack. 

 Trans. Linn. Soc. vol. viii. p. 232. — Rack. Dorset Catalog. 



