184 LAND AND FRESH-WATER MOLLUSKS. 
Shell oval, spiral— 
(a) pillar with an oblique plait. Limncea 
(b) aperture sinistral ... .... Physci 
Shell conical. Ancylus 
Shell discoidal .. .. ... ... ... Planorbis 
Genus Limn^a (Pond Snails ). 
The generic word is derived from the Greek,, 
limne, a marsh or pool. 
The animal is of a greyish colour; the head 
is short and broad, with two flattened short 
triangular tentacles bearing eyes at their bases; 
the foot is broad and short, with two lobes, or 
simply notched in front. The jaw is composed 
of three smooth pieces; the superior one is 
usually produced in front to form a slight beak. 
The general character of the lingual ribbon is 
such as is represented in fig. 24. 
The shell is spiral, oval, or oblong, thin,, 
fragile, and translucent; the last whorl is large; 
the aperture is longer than wide, oval, with a 
thin edge and an oblique fold on the columella. 
They inhabit still and shallow waters, crawl 
slowly, float along the surface of the water with 
the foot, in the fashion of a boat, and the shell 
downwards, for the purpose of supplying them¬ 
selves with air and collecting food. In a state of 
repose they adhere by their foot to stones and 
plants, and are capable of long immersion; in 
