70 GENERA OF SHELLS. 



PiRENA. Shell turreted. Aperture longi- 

 tudinal ; lip sharp, with a sinus at the base and 

 another at the summit. Base of columella 

 curved towards the lip. Operculum horny. 



Principally distinguished from melanopsis by having 

 no callus on the columella, and from that genus and 

 melania by a sinus both at the base and summit of the 

 right lip. 



P. terebralisj spinosa, aurita, granulosa — scalariformis. 



PERISTOMIANA. 



Shell operculated, conoidal or suhdiscoidal : margin of the 

 aperture united. 



Valvata. Shell discoidal or conoid ; 

 whorls cylindrical, not modifying the spiral 

 cavity. Aperture rounded ; margins united, 

 sharp. An orbicular operculum. 



V. piscinalis — tricarinata. 



Paludina. Shell conoid ; whorls rounded 

 or convex, modifying the spiral cavity. Aper- 

 ture round-oval, longitudinal, angular at sum- 

 mit. Lips united, sharp, never curved out- 

 wards. Operculum, round, horny. 



The paludinfe generally live in fresh water, though 

 some inhabit brackish, and even salt water. They are 

 distinguished from the valvatse by the somewhat elon- 

 gated and angular form of the aperture. 



P. Vivipara, achatina, Bengalensis, unicolor, impura, mu- 

 riatica, viridis, — decisa, vivipara, subpurpurea, intertexta, 

 ponderosa, intégra, porata, limosa, decipiens, lustrica, grana, 

 subglobosa. 



