THE SLUG 



213 



The lung-snails (Pulmonates) are either terrestrial, hke the 

 slug, or aquatic. Of the terrestrial pulmonates other than the 

 slug the most important are 

 the snails of the genus Helix} 

 Helix - is noteworthy, because it 

 is richer in species than any 

 other molluscan genus, contain- 

 ing over three thousand of them. 

 The distribution of the genus is 

 world-wide. In North America 

 the snails are most abundant in 

 limestone regions, hence they are 

 less common in New England than in many other parts of the 

 United States. 



Fig. 199. — Shell of Helix alhola- 

 hris, a common forest snail. Nat. 

 size. Photo, by W. H. C. P. 



Fig. 200. — A salt marsh by the edge of the sea, where Mclampus lives. 



The rea.son why Helix contains so many species is because 

 it is undergoing a rapid evolution to-day. That this is so is 



1 helix, a turning round, as in the spire of a snail-shell. ^ Fig. 199. 



