GASTEROPODA. 



193 



umbilicus large; no operculum; many species reversed. 

 Animal, with a stout proboscis-shaped head, having two 

 long tentacula with eyes at the inner 

 base ; foot obtuse behind, and short ; 

 tongue with transverse rows of teeth. 

 — 64 species ; also fossil. 



This genus inhabits lakes and Planorbis cornens. 



ditches in every part of the world; 



many species are common in England, and one or two are 

 found in America with thickened rims ; some of the species 

 are so much flattened that it is difficult to distinguish the 

 upper side from the lower. They are usually thin and 

 fragile, devoid of any bright colour, and do not possess 

 any operculum. 



Philippi places Fleming's genus Segmentina as a sub- 

 genus. 



Camptoceros. Benson, — Shell lengthened, sinistral, 

 with the whorls few, drawn out, but unconnected ; spire 

 projecting, almost erect; the aperture longish, free, and 

 entire ; the lips connected and sharp. Animal, with two 

 blunt, thread-formed feelers, with large eyes between 

 them ; muzzle moderate ; mantle not projecting over the 

 aperture of the shell ; foot short — 1 species, 



C. terebra is brought from Bengal. 



Ancylus. Geoffroy. — Shell oblong, small and trans- 

 parent ; apex rather pointed, and bent to the ^ 

 right ; somewhat resembling a Patella in form, € 

 I but the apex is posterior, whereas that of the Ancylusfiwiata!s , 

 Patella is anterior. Animal, head very large, 

 ; having two large cylindrical tentacula, with eyes at the 

 j base ; foot large. — 5 species ; also fossil. 



The Ancylus is found only in fresh water, in ponds and 



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