106 



C. UNIVALVE Amphibious or Fresh-water 

 Shells, 



LIMNAD^. 



Shell thin, glossy, without operculum ; the margin 

 of the outer lip always acute and never reflected. 



20. Planorbis. Jj 22. Limneiis. 



21. Segmentina. jj 23. Physa. 



XX. PLANORBIS. 



Shell orbicular, flat, disk-like ; the volutions coiling 

 horizontally round each other and visible on both 

 sides ; aperture lateral, oval or crescent-shaped, 

 mostly interrupted by the penultimate volution: 

 operculum none. 



•f The outer volution carinated. 



87. Planorbis car'matus. Shell horn-colour, trans- 

 parent, striate, nearly flat on both sides, with a 

 prominent obtuse keel in the middle. 



