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.
P. vortex.
P. deformis.
P. contortus.
P. hispidus.
P. nitidus.
P. imbricatus.
_P. carinatus._ The keeled Planorbis.
Species with a keel; depressed, upper side concave.
_P. corneus._ The horny Planorbis. Pl. 20, fig. 4.
Species without a keel.