PULMONATA. 109 



referred to a distinct species. It certainly presents a general resemblance to P. lens; 

 but the upper side is more arched, the under side flatter, the whorls enlarge more 

 rapidly, and the margins are more compressed and more acutely carinated than in 

 that species ; the umbilical cavity, also, is not so deep, and the aperture is of a more 

 oblique and of a more elongated heart shape. 



In the rapidly increasing size of the whorls and the condition of the marginal keel, 

 the present species presents an analogy with the recent P. exacutus (Gould) ; but in 

 the latter shell, the upper sides of the whorls are not concealed, the under sides are 

 more convex, the umbilical cavity is deeper, and the aperture is almost obcordate. In 

 P. tropis the shell is more compressed, the whorls enlarge more slowly, and the umbi- 

 lical cavity is wider. 



Size. — Diameter, 2-10ths of an inch. 



Locality. — Sconce. 



Planorbis cylindricus. — Mr. Sowerby has described a shell under this specific 

 name, (Min. Conch., vol. ii, p. 90, t. 140, fig. 2,) the distinguishing characters of which 

 are the vertically flattened, adpressed volutions, concentrically striated on the under 

 surfaces, and the oblong quadrangular aperture. I have not met with any specimen 

 which presents these characters ; and as the original specimen, unfortunately, has been 

 broken, I cannot give any description or figure of the species. It is not improbable 

 that the shell described by Mr. Sowerby was a fragment, consisting of the early volu- 

 tions of one of the larger species I have described, possibly of P. rotundatus, in which 

 the whorls, in the young state, are somewhat adpressed and the aperture is 

 subquadrate. 



Genus 14th. Ancylus.* Geoffroy. 



Ancylls, Geoffroy, 1707; Mailer, 1774; Draparnaud, 1805; De Roissy, 1805; 



Ferussac, 1819; Lamarck, 1820; Blainville, 1825; Guildiny, 1821. 

 Patella (spec), Linnaeus, Bruyuiere, Montayu. 

 Helcion (spec), Montfort, 1810. 

 Ansulus vel Ansylus, Gray, 1840. 



Gen. Char. — Patelliform, thin, obliquely conical, sinistral ; apex rather pointed, 

 compressed, not lengthened nor spiral, turned sidewise towards the right margin and 

 backwards, not marginal : aperture oval or oblong, margins simple. 



This genus, first withdrawn by Geoffroy from the Patella, was rejected both by 

 Linnaeus and Bruguiere, but was revived by Draparnaud, and placed near the Limnaida, 

 on account of the similarity between the animal and those of Limnaa and Planorbis. 

 Ferussac, who had noticed the occasional ascent of the animal to the surface of the 



* Etyni. AyKvXos, crooked, twisted. 



