ALONG THE STREAM 245 



some distance from the sea. In the Philippines 

 are some species of this genus (Neritodryas, from 

 Nereis, a sea nymph, and Dryas, a tree nymph) 

 which live on trees at a distance oj a quarter oj a mile 

 from the ocean! The genus was probably derived 

 from Nerita, a very similar group that is wholly 

 marine. 



Farther upstream where the water is entirely 

 fresh one finds a variety of small moUusks in the 

 sandy muddy bottom; several species of Plan- 

 orbis, with their flat, closely coiled shells, so that 

 there is a depression at both the spire and base. 

 There is a related snail living in the upper reaches 

 of the streams the shell of which resembles Plan- 

 orbis and is likely an aberrant member of that 

 genus. It has been called by several generic 

 names but is generally known as Ameria scalaris. 

 In some cases the shell is disk-shaped like Plan- 

 orbis, in others it looks as though the spire had 

 been awkwardly pushed up when in a plastic 

 state; there is every variation between extreme 

 forms. They grow by millions in the Everglades 

 and scarcely any two are exactly alike. 



Still another interesting fresh-water mollusk is 

 found in the streams of Lower Florida. It is an 



