FAMILY LYMN^ACEA. 137 



eastern and south-eastern counties ; and the little P. nitidus {Seg- 

 mentina) common in the neighbourhood of London, is a South-Eng- 

 land species, and ranges little south of the latitude of England on 

 the Continent. Planorbis inhabits all kinds of stagnant pools and 

 ditches and gently running brooks, chiefly adhering to flags and 

 other water-plants. When left dry in the bed of a stream by the 

 retiring of the water, the animal encloses itself within the shell 

 by an epiphragm. 



The British species of Planorbis are : — 



1. corneus. Shell large and ventricose, of five to five and a half 



rudely striated faintly malleated whorls. 



2. albus. Shell small, rather depressed, of four and a half to 



five longitudinally thread-striated whorls. 



3. glaber. Shell very small, smooth, glossy, convex above, rather 



concave below, of four to four and a half rounded whorls. 



4. crista. Shell very small, Nautiloid, of only three whorls, 



covered by a membranaceous epidermis, which is puckered 

 and prolonged at the periphery into lashes. 



5. carinatus. Shell moderate in size, very depressed, of five 



slowly enlarging whorls, keeled externally a little below the 

 centre. 



6. complanatus. Shell larger, very depressed, of five slowly en- 



larging whorls, keeled externally at the basal edge. 



7. vortex. Shell rather small, very thinly depressed, of six to 



seven extremely slowly increasing whorls. 



8. spirorbis. Shell small, very depressed, of five to six slowly 



increasing faintly two-angled whorls. 



9. contortus. Shell rather small, depressed, of six to seven 



broader extremely closely coiled whorls. 



10. fontanus. Shell minute, amber horny, of three and a half to 



four whorls convexly sloping on both sides towards the 

 periphery. 



11. nitidus. Shell minute, transparent amber horny, of three and 



a half to four whorls sloping on the upper side towards the 

 periphery, divided internally at intervals by septa. 



