PULMONATA. 97 



No. 50. LiMNiEA tenuis. F. R Edwards. Tab. XIV, fig. 11 a — b. 



L. testa tenui, ovato-ventricosd, sub-turritd ; anfractibus sex, convexis, ultimo magno ; 

 apice acuto : aperturd ovatd, effusd, in longitudinern bessem totius testa fere cequanti; 

 plica columellari brevi, angustd, rotundatd, parum tortuosd. 



This species presents so close an analogy with L. tumida, that it would be difficult 

 to separate the two, if it were not for the great difference in size. The shell is very 

 thin, ovate, and formed of six convex volutions, depressed round the suture, which 

 character imparts to it a sub-turreted appearance ; the spire is somewhat elevated and 

 pointed ; the body-whorl large and ventricose : the aperture ovate, effuse, and nearly 

 as long as two thirds of the entire shell ; the columellar fold short, narrow, rounded, 

 and not much twisted. 



Size. — Axis, 8-10ths of an inch; diameter, 4-10ths of an inch. 



Locality. — Headon Hill. 



Genus 13th. Planorbis. Geoffroy. 



Coketus, Adanson, 1757. 



Planorbis, Geoffroy, 1767; Guettard, 1770; Miiller, 1773-4. 



Gen. Char. — Shell discoidal, spire depressed ; volutions apparent above and below, 

 convoluted upon a nearly horizontal plane, thin, generally smooth, ventricose, some- 

 times carinated : aperture simple, lunate, crescent-shaped or sub-quadrate, impinged 

 upon by the preceding volution ; outer lip generally thin, sharp edged, sometimes 

 thickened or reflected ; columellar lip slightly spreading over the body whorl. No 

 operculum. 



The animals belonging to this genus were placed by Linnseus among the Helices ; 

 they had, however, been separated, as a distinct group, by Lister, nearly a century 

 previously, and formed the third section of the Lacustrine shells of that author. 

 Shortly before the publication of the Systema Naturae, Adanson described a small 

 species to which he gave the generic name Coretus. The genus was afterwards defined 

 by Geoffroy under the present name Planorbis ; and Miiller, to whom it has been gene- 

 rally attributed, only adopted Geoffroy's name. 



The animal of Planorbis is elongated, slender, and strongly rolled up; the head is 

 furnished with two long contractile tentacles, at the internal bases of which the eyes 

 are placed ; the orifices are on the left side ; the organs of generation distinct. 



Whether the shell of Planorbis is dextral or sinistral is a question which has been 

 much discussed, and, by some authors, is considered as still undecided. By 

 Linnseus, Miiller, and subsequent writers to the time of Cuvier, it was regarded as 

 dextral, and was described as supra umbilicata. The transposition in P. corneus of the 



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