PLANORBIS.
"The genera are,
| Planorbis | Helix. |
| Physa | Bulla. |
| Lymnæa | Helix. |
"Planorbis is a discoid shell, and one peculiarity of the genus is, that they are all reverse shells. In a discoid shell the spire is depressed; when held up, the whorls turn from right to left, and the aperture is left-handed. The largest species is P. cornu-arietis,[B] which is a native of Brazil. P. corneus is common in ponds and ditches. Empty shells are to be found at the edge of the water. ([Plate 9.]) If you take the animal to examine, and study its habits, remember that you have no right to injure it, and that you have already promised me that no kind of cruelty shall take place.
[B] Ram's-horn.
"P. vortex is a smaller species. The outer valve is carinated.
"Physa is found in fresh-water upon aquatic plants. They are small shells.
LYMNÆA.
MELANOPSIS.
VALVATA.