FAMILY VIII.
Lymnæcea. Three genera.
1. Planorbis. Twelve species.
Taken from the Helix of Linnæus to distinguish the aquatic from the terrestrial shells. This genus is found in fresh water, and has no operculum.
Shell thin, often sinistral, discoid, or involuted almost in the same vertical plane; the spire not projecting and entirely lateral, so that the shell is hollowed or depressed on each side; aperture small, transverse, with edges sharp, not reflected, disunited by the last whorl of the spire which modifies it; sometimes carinated.
Planorbis cornu-arietis.
P. corneus.
P. carinatus.
P. lutescens.
P. orientalis.
P. spirorbis.