10 



23. PiiYSA. Shell reversed, oval or oblong : aper- 

 ture longitudinal ; the pillar simple. 



Tij2)e. Bulla Hypnorum, Montagu. 



e. Peristomadce. 



Animal inhabiting fresh waters. Shell furnished with 

 an operculum : aperture circular ; the margin uni- 

 ted all round. 



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. Hehx vivipara, Montagu. 



f. Nentidce. 



Animal inhabiting fresh water. Shell somewhat glo- 

 bular or semioval : aperture half divided trans- 

 versely, furnished with an operculum. 



