FAMILY PERISTOMAL. 185 



aperture entire, from the Lymnceacea, which have no operculum, 

 and whose shell is formed of whorls resting so far one upon the 

 other, that the margins of the aperture are divided. Along with 

 this similarity in the structure of the shell, there is a characteristic 

 similarity in the animal. The head is in each case produced into a 

 ringed proboscis, and the foot is furnished with an operculum. 

 But there are important generic differences. Bythinia has the ten- 

 tacles thread-like and flexible, with the eyes at their outer base, on 

 sessile swellings, the operculum being sometimes concentric, some- 

 times paucispiral. Paludina has the tentacles rather stout and 

 cylindrical, with the eyes raised on stalks to make room for a pair 

 of lobes, of which the right-hand one is folded into a tube, in place 

 of a simple orifice, for conveying water to the branchial chamber. 

 Valvata has the eyes at the inner base of the tentacles, and the 

 branchiae are external, in the form of a plume of spiral filaments. In 

 the intertropical parts of both hemispheres there is a genus belong- 

 ing to this family, Ampullaria, larger in dimensions than either of 

 the foregoing, and more abundant in species, of which we have no 

 representative in Britain. The Bythinice and Paludince are scat- 

 tered throughout the Eastern Hemisphere, excepting Australia; and 

 they are comparatively plentiful in North America. Valvata is 

 confined to the temperate and north-temperate latitudes of the Old 

 World, and it occupies the same latitudes in the New World, 

 reaching southwards to the West Indies. The Peristomata are 

 essentially of mud-dwelling habits. Many of them, when exposed 

 for hours daily on the river's brink at the fall of the tide, will bur- 

 row into the mud and remain embedded until its return. We have 

 seven species in all. 



The British genera of Peristomata are : — 



1. Bythinia. Animal with flexible filiform tentacles, having the 



eyes placed externally at their base on sessile swellings, carry- 

 ing a conically turbinated shell, of which the operculum is 

 sometimes concentric, sometimes paucispiral. 



2. Paludina. Animal with cylindrical tentacles, having the eyes 



raised externally on stalks, to make room for a pair of lobes, 

 of which the right one forms a branchial siphonic tube. 



3. Valvata. Animal with rather slender tentacles, having the 



eyes at their inner base, and the branchiae external in the 

 form of a plume of spiral filaments. Shell sometimes tur- 

 binated, sometimes discoid. 



