140 BRITISH MOLLUSKS. 



they are covered with a slight hairy epidermis. The striae do not, 

 however, become obsolete or irregular with age ; they present a 

 neatly defined sculpture throughout, and on the periphery of the 

 outer whorl the central stria assumes a prominence inclining to the 

 magnitude of a keel. The spire, as seen on the upper disk of the 

 shell, is only moderately immersed. On the lower disk the whorls 

 form an even superficially concave umbilicus. 



The animal of P. albus varies in colour from grey to reddish or 

 yellowish-brown. It is common on water-plants in all parts of the 

 British Isles. Mr. Macgillivray describes having collected it abun- 

 dantly in the Aberdeen Canal, on different species of Potamogeton, 

 and ascertained, by keeping specimens for many days ahve, that it 

 fed on that plant with great voracity. " The animal," he adds, 

 "in walking in the water, bears the shell inclined at an angle of 

 from seventy to eighty degrees. Out of the water, it drags the shell, 

 laid flat, by sudden jerks." Abroad it has the same extended range 

 as the preceding species. 



3. Planorbis glaber. Smooth Planorbis. 



Shell ; rather convex, depressed above, concave below, thin, glossy, 

 greyish horny, sometimes iridescent, whorls four 

 to four and a half, rounded, smooth ; aperture ob- ^ 



liquely lunar-rounded. 



Planorbis glaber, Jeffreys (1830), Trans. Linn. Soc. vol. xvi. 

 p. 387. 



Planorbis lesvis, Alder (1830), Trans. Nat. Hist. Soc. North- 

 umb. vol. ii. p. 337. 



Hab. Throughout Europe. North Africa. Madeira. (On 

 water-plants in marshes, lakes, and ponds.) 



This little glossy species, unnoticed by Continental writers, has 

 been well observed by Mr. Jeffreys. It is obviously distinct from 

 P. albus, with which species it is the nearest allied. The whorls 

 are tumidly rounded and smooth, and the shell is uniformly small. 

 Mr. Jeffreys thinks that P. cornu of Ehrenberg, P. Possmasleri 

 of Auerswald, and P. gyrorbis of Seckendorf, are referable to P. 

 glaber. The animal is described by the same author as being 

 yellowish-grey, with rather short cylindrical tentacles, ending in a 

 blunt point ; foot rather broad, especially in front. 



