THE MOLLUSCA 



981 



?6 (27) Shell thin; spire short, acute; body- whorl large, inflated; lip expanded. 



Subgenus Radix Montfort. 



The typical species, Lymnaea auricularia L. (Fig. 1420), is European, 

 but has been locally introduced in several of the Eastern States. 



Fig. 1420. 



27 (28) Shell thin; spire short; body-whorl large, elongated, not inflated; sur- 

 face sculptured with spiral incised lines. 



Subgenus Pseudosuccinea Baker. 



The typical species, Lymnaea columella Say, (Fig. 1421), has a general distribu- 

 tion throughout the eastern United States and Canada. 



Fig. 1421. 



28 (29) Shell very long and slender; spire elongated, acute; body-whorl long 

 and narrow; columella smooth. Subgenus Acella Haldeman. 



A single species, Lymnaea holdemani "Desh." W. G. Binn., (Fig. 1422), occurs 

 in the St. Lawrence drainage system and the upper part of the Mississippi River. 



Fig. 1422. 



29 (30) Shell varying from elongate to short ovate; outer lip (usually) some- 

 what thickened within; columella somewhat twisted and 

 plicate; surface with strong, spirally impressed lines. 



Subgenus Stagnicola Leach. 



The typical species, Lymnaea palustris Mull., (Fig. 1423), is a circum- 

 boreal and is usually the most common species in the Northern States and 

 Canada. 



Fig. 1423. 



30 (31) Shell as in Stagnicola, but with the surface longitudinally costate. 



Subgenus Polyrhytis Meek. 



The only recent species known, Lymnaea utahensis Call., (Fig. 1424), is from 

 Utah. 



Fio. 1434* 



