MOLLUSC A OR UNIVALVES. 183 



50. Physa. — Shell oval or oblong, smooth 

 and thin ; aperture contracted above ; 

 inner lip smooth, outer very thin; 

 no operculum ; shell generally re- 

 versed. Animal having two long ten- 

 tacula, with eyes at the base; foot 

 long; mantle large, so as to cover part of the 

 shell, and very transparent. — 10 species recent, 

 and 1 fossil. 



These animals are most frequently found on 

 the under side of the leaves of aquatic plants : 

 they have a very singular way of adhering to 

 the surface of the water with the shell down- 

 wards, and crawl in that direction with as much 

 apparent ease as on a solid surface, and they 

 will occasionally let themselves down gradually 

 by a thread. This power of crawling under 

 water against its surface is not wholly con- 

 fined to this and the following genus. Many 

 species are found in Europe, others in Africa, 

 America, &c. 

 <? 51. Lymnea. — Shell oval, inflated, thin; 

 spire small and pointed ; aperture oblong, 

 sometimes very large ; occasionally a fold on 

 the columellar lip; no operculum. Animal 

 N 4 



