192 



POPULAR CONCHOLOGY. 



by the very large mantle which covers the shell. — 4 or 

 5 species. 



From the Philippines, Germany, and Holland. 



Physa. Draparnaud. (Bulinus Adan.) — Shell oval 

 or oblong, smooth and thin ; aperture contracted - 



part of the shell, and very transparent. — 14 

 species ; also fossil. 



These Molluscs are most frequently found on the under 

 side of the leaves of aquatic plants : they have also a very 

 singular way of adhering to the surface of the water with 

 the shell downwards, and they crawl in that direction 

 with as much apparent ease as on a solid surface, occa- 

 sionally letting themselves down gradually by a thread. 

 This power of crawling under water against its surface 

 is not wholly confined to this genus. Many species are 

 found in Europe, others in Africa, some in California, 

 and other parts of America. 



There are two British species, P. fontinalis and hyp- 

 norum. Beck divides this genus into Aplexa of Fleum. 

 and Bulinus. 



Physopsis. Krauss. — Shell sinistral, umbilicated, 

 thin, egg-shaped ; the aperture longitudinal ; the outer 

 lip notched, the inner twisted, plaited and truncated at 

 the base ; no distinct columella edge. — 1 species. 

 From Southern Africa. 



Planorbis. Midler. (Coretus Adan.) — Shell flat 

 and disk-shaped; mouth semicircular, and removed from 

 the axis of the shell; whorls apparent on both sides; 



nalis. 



