PHYSA. 99 



wJiorh angular towards the suture, the middle one rather 

 more prominent than the penultimate whorl, causing the 

 summit of the spire to appear abruptly terminated. 



Var. 2. ciirta. Spire extremely short. Bulla jiuviatilis, 

 Turt. Conch. Diet. p. 27. 



Yar. 3. ohlonga. Spire considerably produced. 



Yar. 4. alhina. Shell of a milk-white colour. 



Habitat : On water-cresses and other aquatic plants 

 in running brooks, as well as in slow rivers, canals, and 

 ditches everywhere in Great Britain, as far north as 

 Aberdeenshire. Y'ar. 1. Dublin (Humphreys and War- 

 ren). Var. 2. Clonoony Barracks, Ireland (Brown) ; 

 Bramerton, Norfolk (J. G. J.). Var. 3. Anglesea, on 

 Chara aspera (J. de C. Sowerby) ; Naas, Ireland (Hum- 

 phreys). Var. 4. Birkenhead (Webster). This species 

 is widely diffused on the Continent, and ranges from 

 Finland to Sicily. 



This common and pretty little mollusk is rather lively, 

 creeping and floating by jerks. A considerable por- 

 tion of the shell (especially the back of the spire) in its 

 living state is often covered with the spores of Con- 

 fervas or some of the freshwater Algse, which shows 

 that the mantle does not envelope all the surface. When 

 the fry are excluded from their gelatinous case, they are 

 about the size of a pin^s head, and are very active. The 

 jerking motion which this animal has, is said to be owing 

 to its being infested by a small kind of parasitic worm 

 which causes it some uneasiness. I should rather be in- 

 clined to attribute this motion to the length and narrow- 

 ness of the foot, which has to support a comparatively 

 bulky shell. According to Montagu, the P. fontinalis 

 spins a filament by which it lets itself down to the bottom 

 after floating, if there is no leaf or stalk near it. Leach 

 says that when it is annoyed by the approach of wander- 



r2 



