PLANORBIS. 



GENERIC CHARACTER. 



Shell univalve, thin, fragile, discoidal or revolving in 

 the same horizontal plane ; concave above and beneath, 

 the spire, being impressed ; aperture rounded-ovate, en- 

 tire at base, the labia interrupted by the convexity of the 

 penultimate volution ; labrum simple ; operculum none. 

 Animal depressed spiral sinistral ; foot rounded, small ; 

 tentacula two, long, slender, contractile ; eyes at the in- 

 ner base of the tentacula ; mouth someviiiat cxertile, 

 armed above with a lunate, simple tooth, and beneath with 

 small cartilaginous hooks ; pulmonary orifice on the col- 

 lar, near that of the vent ; genitals on the same side, and 

 separate, that of the male near the tentaculum, of the fe- 

 male at the margin of the collar. 



OBSERVATIONS. 



Lister first separated these shells by placing them in a 

 distinct section of the fluviatile kinds. In 1756 Guettard 

 characterized the genus very accurately, both the animal 

 and its shell, and applied to it the name by which it isnov^r 

 universally known. Adanson and Gcoffroy afterwards 

 adopted the genus, the former under the name of Goret. 

 Although this natural genus appeared to be thus firmly es- 

 tablished, our great master Linne subsequently placed the 

 species in the genus Helix, without any distinction what- 



Pl, £4. 



