9 



d. Limnadce. 



Animal amphibious or inhabiting fresh waters, with 

 the tentacula flattened. Shell glossy, without 

 operculum ; the margin of the outer-lip always 

 acute, and never reflected. 



20. PLANORBIS. Shell orbicular, flat, discoid; the 

 volutions horizontally convolute, and all of them 

 visible on both sides : aperture transverse, ovate 

 or lunate. 



Type. Helix cornea, Montagu. 



21. SEGMENTINA. Shell orbicular, somewhat dis- 

 coid : aperture oval, semiconcamerate within ; the 

 partitions transverse, and resembling a triradiate 

 opening. 



Type. Nautilus lacustris, Montagu. 



22. LIMNEUS. Shell regular, spiral, oblong-oval or 

 turrited : aperture longitudinal, mostly dilated ; 

 the pillar twisted. 



Type. Helix stagnalis, Montagu. 



