10 



3. PHYSA. Shell reversed, oval or oblong : aper- 

 ture longitudinal ; the pillar simple. 



Type. Bulla Hypnorum, Montagu. 



e. Peristomadtz. 



Animal inhabiting fresh waters. Shell furnished with 

 an operculum : aperture circular ; the margin uni- 



24. VALVATA. Shell subconic or discoid : aperture 

 exactly orbicular : operculum marked with a single 

 spiral line. 



Type. Turbo fontinalis, Montagu. 



25. PALUDINA. Shell conoid or oblong : aperture 

 roundish-oval, slightly angular at the top : oper- 

 culum roundish, marked with concentric striae. 



Type. Helix vivipara, Montagu. 



f. Neritidce. 



Animal inhabiting fresh water. Shell somewhat glo- 

 bular or semioval: aperture half divided trans- 

 versely, furnished with an operculum. 



