SNAILS AND MUSSELS 



to be flattened and to join the body at a sharp angle. Length 

 one-half to three-quarters of an inch. Many references to 

 P. heterostropha apply to this snail (Baker, Bibliography, 

 p. 421). 



Occurrence. — Common in eastern states. 



Wheel-snails, Family Planorbidae. — These are air-breathing 

 snails without an operculum whose shells are twisted right 

 or left in a flattened coil, with the spire sunken in the center. 



Fig. 249. — Three-coiled snail, Plajiorbis trivolvis: 

 I, animal expanded (after Baker); 2, -shell coils; 3, 

 shell showing body aperture. 



Three-coiled snail, Planorbis trivolvis (Helisoma trivolvis 

 of Baker). — The shells are transversely lined and thin lipped. 

 Width of shell, about five-sixths of an inch. Typical trivolvis 

 lives in protected places, bays, pools, marshes, and swamps, 

 where the vrater is shallow. It is one of the larger wheel- 

 snails; this and its expanded bod}^ aperture (Fig. 249) help to 

 identify it. 



311 



A 



