PLANORBIS. 83 



HABITAT : On aquatic plants in marshes, lakes, ponds, 

 and ditches, from the northernmost extremity of Zetland 

 to the Channel Isles. It is also one of our upper tertiary 

 fossils. The variety is not uncommon, and merges in- 

 sensibly into the ordinary form. The degree of sculpture 

 appears to depend much on age, as it is usually stronger 

 in half-grown individuals and disappears in the adult. 

 A monstrosity also occurs in which the whorls are more 

 or less twisted and separate from each other, sometimes 

 being raised like a snake lying on its coil. The range of 

 this species abroad extends from Finland to the Pyre- 

 nees and even to Algeria. 



This pretty little mollusk is slow in its movements, 

 and may be noticed feeding on the decaying leaves of the 

 Iris pseudacorus and water-plants. According to Bou- 

 chard-Chantereaux, it lays only from 3 to 6 eggs, which 

 leave the capsule in ten or twelve days. The sculpture 

 of the shell is extremely elegant ; and it is by far the 

 smallest of its kind. The minuteness of its size, dull 

 appearance, and comparatively large umbilicus will at 

 once serve to distinguish it from either of the foregoing 

 species. If the rings which encircle each whorl are 

 marks of annual growth, it must attain a very respectable 

 old age for a mollusk, as I have counted as many as 20 

 rings in one specimen. In all probability, however, these 

 marks do not indicate the annual, but only the periodical 

 growth, several of them being formed in the first year. 



4. P. AL'BUS*, Muller. 



P. albus, Mull. Verm. Hist. pt. ii. p. 164 ; F. & H. iv. p. 149, pi. cxxvi. 

 f. 1, 2. 



BODY grey or dirty-brown, sometimes inclined to a reddish 

 hue, with fine but indistinct black specks : head thick, rounded 



* White, 



