182 POPULAR CONCHOLOGY. 



the air on its surface. The shell is spiral, with 

 the exception of one genus ; the external sur- 

 face generally smooth, and the right margin of 

 the aperture always sharp and not reflected. 

 They do not possess an operculum. There 

 are four genera. 



49. Planorbis. — Shell flat and disk-shaped ; 

 mouth semicircular, and 

 removed from the axis 

 of the shell ; whorls ap- 

 parent on both sides ; 

 umbilicus large ; no operculum ; many species 

 reversed. Animal having two very long ten- 

 tacula, with eyes at the base ; foot oval and 

 short. — 22 species recent, and 9 fossil. 



This genus inhabits lakes and 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 ; 

 but Deshayes says it may generally be done by 

 observing that the upper part of the lip is more 

 advanced than the lower. They are usually 

 thin and fragile. 



