260 PNEUMONOBRANCHIATA. 



In stagnant waters, on aquatic plants. 



Animal greyish. (Sturm, t. 42.) 



Shell about a quarter of an inch in diameter, very 

 thin and brittle, pale horn-colour, marked with very 

 fine close-set raised circular striae, which are clothed 

 with deciduous bristles, and crossed with obscure 

 longitudinal lines ; volutions five, the first very large 

 and rounded ; the upper surface a little sunk in the 

 middle, the under side more strongly concave ; aper- 

 ture roundish-oval, dilated, higher than wide, with 

 the upper angle much produced. 



When quite fresh, this beautiful species is clothed 

 with a fine velvety pile composed of short points 

 seated on the raised concentric striae, and which fall 

 off with the epidermis ; and in its depilated state 

 may be the Helix spirorbis of Linne, as he no where 

 else mentions so very common a species, Gmelin hav- 

 ing probably quoted it twice, both as H. spirorbis 

 and H. alba. In this state it answers well to his 

 character of H. spirorbis, in the Fauna Suecica, " testa 

 utrinque concava, plana, albida : anfractibus quinque 

 teretibus." 



The Helix Somershamensis Sheppard (Linn. Trans. 

 xvi. 159.), described as a land shell found on old de- 

 cayed wood, is said much to resemble this species in 

 shape and appearance. It requires further examin- 

 ation. 



Mr. Alder observes, " The examination of Mr. Jef- 

 freys* specimen of P. olaber confirms me in the opinion 

 that it is a variety of this species. In some speci- 

 mens of P. albus the spiral striae are scarcely discern- 

 ible even in a living state, and become quite obliter- 



