AND THEIR INHABITANTS. 215 



irregularly striate by curved lines of growth marked 

 with fine and close-set spiral striae, more perceptible 

 in the first whorls ; the upper surface is pitted or 

 impressed irregularly like cut-glass; the length is 

 0-35, the breadth i. The variety albin us has a white 

 shell. It is local, and occurs in many parts of 

 England and Ireland. It is larger than any other 

 European species of planorbis. When irritated it- 

 emits a purple fluid from its neck. On a warm day 

 it floats on the surface of the water. It lays two or 

 three capsules, each containing from twenty to forty 

 eggs, which are excluded at the end of fifteen or 



PLANORBIS CORNEUS. 



sixteen days. The epidermis of the young shell is 

 covered with fine down ; its surface resembles velvet 

 pile. P. contortus has a black body, tinged with 

 red. The tentacles are slender, and the eyes small ; 

 the foot is broad, narrowing to a blunt tail The 

 shell is flat, with a deep depression or concavity in 

 the middle, very concave below. The epidermis is 

 thick; the length is 0-075; tne breadth, 0-175. 

 The variety albinus is nearly white. Its habitat is on 

 water-plants in fresh ponds and ditches throughout 

 Britain, as far north as Scotland, but it is local. 

 The Physa hypnorum has a lustrous dark gray, 



