252 PNEUMONOBRANCHIATA. 



In rivers and streams, on aquatic plants. 



Animal blackish grey ; tentacles paler. 



Shell nearly half an inch long, and half as much 

 broad, very thin and fragile ; spire extremely short, 

 of four volutions, the lower one much inflated, the 

 others small, and ending obtusely ; aperture covering 

 nearly the whole of the shell ; pillar slightly sinuate 

 and white, not reflected. 



This shell varies considerably in shape. Mr. Jef- 

 freys distinguished four varieties. Some have the spire 

 elongated considerably more than the rest. Dr.Turton 

 (Conch. Diet.) described a small subglobose specimen 

 under the name of Bulla Jluviatilis, but it probably is 

 only a young specimen of the common state. 



It is the young specimens of these shells alone 

 which agree with the Linnaean and Lamarckian cha- 

 racter of the species, in the shortness of the spire ; for, 

 as the shell increases in size, the whorls are gradually 

 turned more obliquely down the axis, so that the older 

 shells have a longer spire in proportion than the young 

 ones. 



There are nevertheless two very distinct varieties, 

 which may prove to be distinct species, and indeed 

 have been considered so by several of my friends, as Mr. 

 Fryer and Mr. Hinch, who study these animals. The 

 one which agrees best with Linnaeus's and Lamarck's 

 character of Physa fontinalis, is generally a small shell, 

 of a clear yellow colour, with a very short rounded 

 spire, formed of 3J or 4 very gradually enlarging 

 whorls, the suture of the last being more oblique than 

 the rest, and with a subacute tip. It is a young 

 shell of this variety that was most probably figured 

 by Donovan under the name of Helix bullceoides 



