FIELD BOOK OF PONDS AND STREAMS 



in streams as well as in quiet ponds. They are common on 

 vegetation; Ancylus parallelus (Fig. 253) is often found on the 

 submerged stems of sedges in pond shallows, but A . rivularis 

 (Fig. 254) clings to stones in lively currents. Limpets are 

 generally distributed. 



Family Amnicolidae. — These are small snails with shells not 

 more than half an inch long, and right-handed. The animal 

 has an oblong foot truncated in front and rounded behind (Fig. 

 255) ; its right gill is the only one developed. These snails 

 lay eggs, sometimes deposited on the shells of other snails in 

 clusters of eight or ten, as in the genus Biilimus, or singly as 

 in Amnicola. Snails of this family are common in shallow 

 water, on sandy bottoms, but also on pondweeds and other 

 vegetation. Widely distributed and common. 



Fig. 256. — Shell of Goniobasis. 



Fig. 257. — Valvata tricarinata: i, animal (after 

 Baker); 2 shell, showing keeled edge of whorls; 3, 

 shell, showing operculum in the body aperture and 

 cavity in the base of the spire; 4, shell from the side, 

 showing the blunt apex of the spire. 



314 



