"The forms of the species in _Cyclostoma_ are variable; but the aperture is circular, and the margin revolute, or rolled back. _C. elegans_ is often to be found on hedge-banks or chalk soils. It is a pretty shell, sometimes white, often tinted with purple. "In the family _Lymænana_, the species are amphibious; inhabiting fresh-water; but rising to the surface to breathe the air. They have but two tentacula. "As several species are British, you may have the satisfaction of examining them for yourselves. * * * * * [Sidenote: 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. [Sidenote: LYMNÆA. MELANOPSIS. VALVATA.] "The animal of the _Lymnæa_ genus has two flat tentacula. _L. stagnalis_ is a very pretty spiral shell, common in ponds. "_L. auricularia_ is also frequent. It is much smaller than the first species. The last whorl is swelling, and the aperture very wide. They are both thin and brittle. * * * * * "The family _Melaniana_ are chiefly foreign. The shells are covered with a dark-coloured epidermis. They are operculated. _Melania_ _Helix_. _Melanopsis._ _Pirèna._ "A species of _Melanopsis_ inhabits the river Orontes, in Syria. * * * * * "There is yet another family connected with _Helix_, the _Peristomata_, containing _Valvata._ _Paludina_ _Helix_. _Ampullaria_ _Helix_, partly. [Sidenote: PALUDINA.] "Some of the _Valvata_ genus are found in fresh-water in Britain and other European countries. The shells are small; they are discoid or conoid, and have an operculum. In the shells of this family the margin of the aperture is carried completely round. In _Paludina_ the whorls are convex. They generally inhabit fresh-waters. "_P. vivipara_ is found in rivers. Quantities of empty shells may be taken from the sand of the Thames. "Fossil species abound--Petworth marble is full of them." CHAPTER X. [Sidenote: NERITACEA. NERITINA.]