The Shell-Collector’s Handbook. 4] 
or placed on pedicels at the base of the tentacles ; 
gill internal. 
2. Paludina, Lam.—Animal ovoviviparous ; eyes 
placed on pedicels ; operculum horny. 
3. Bythinia, Gray.—Animal oviparous; oper- 
culum shelly. 
4. Hydrobia, Hartm.—Operculum horny, thin, 
paucispiral ; eyes placed on tubercles. 
Famity III.—Valvatide. 
Shell conoid, more or less depressed ; mouth 
circular ; operculum horny, multi-spiral. Body spiral, 
with two tentacles, and provided with a long, plume- 
like gill, protruded when the animal is crawling ; eyes 
situated on the inner-side of the base of the tentacles. 
5. Valvata, Mill. 
Orpver [I.—PUuLMONOBRANCHIATA. 
Shell generally spiral and external, but sometimes 
(Limacide) rudimentary and internal, or wanting. 
Body spiral, generally non-operculated, but sometimes 
with an operculum ; respiration effected by means of 
a lung. 
Famity I—Limneide. 
Shell spiral or hood-shaped; mouth edentulous. 
Tentacles two; eyes sessile. 
6. Planorbis, Guettard.—Shell orbicular, flat, 
and coiled nearly in the same plane; mouth semi- 
circular ; umbilicus distinct; non-operculated. Ten- 
tacles two in number, very long; eyes sessile; foot 
oval and short. 
