"The third order of _Mollusca_, _Trachelipoda_, begins with a well-known genus, the snail, _Helix_. The term signifies that the _foot_ is situated under the neck, or anterior part of the body. The families in this order are numerous: they are divided into two sections; the first includes those that breathe only in the air; the second those that can exist only in the water, and are furnished with a syphon. * * * * * [Sidenote: HELIX. CUROCOLLA. ANOSTOMA.] "First section: family _Colimacea_; genera numerous; animals live upon land only; tentacula generally four; during winter they enclose themselves in their shells, with a false operculum. _Helix_, snail _Helix_. _Carocolla_ _Helix_. _Anostoma_ _Helix_. _Helicina_ _Helix_. _Pupa_ _Helix_. _Clausilia_ _Helix_. _Bulimus_ _Helix_. _Achatina._ _Succinea_ _Helix_. _Auricula_ _Voluta_. _Cyclostoma._ "What a number of new genera!" said Lucy. "I see the forms of the shells vary very much; and how beautiful these little transparent shells are!" "They will find a place shortly. Here is a well-known species, _H. aspersa_, in most of its varieties; _H. pomatia_, the apple-snail, now naturalized in the county of Surrey; _H. ericetorum_, white with brown bands, very frequent on chalky soils; _H. citrina_, transparent, pale yellow, sometimes with one dark band; _H. muralis_, from the walls of Rome; _H. bidentalis_, from Teneriffe; and the little _Helix hispida_, which you may search for in your own garden; it is small, dark brown, and rough. "_Carocolla_ has the shells more flattened than _Helix_. "_Anostoma depressa_ is a rare and curious shell. "_Helicina_ is a West Indian genus. You saw them just now. We shall find Helix in two other families. [Sidenote: PUPA. BULIMUS. AURICULA.] "_Pupa_ is a curious genus. The shells resemble a chrysalis. A few minute species are found in Britain. _P. muscorum_ I have found buried among damp moss. The larger species are natives of tropical regions. These shells are often found _decollated_. "_Clausilia papillaris_ is a pretty shell. (Plate 9.) "_Clausilia rugosa_ is found in some parts of Britain, under old hedges, at the foot of old trees, and similar places. It is a tapering shell, with the aperture reversed, or left-handed, and bi-dentated: the colour red-brown. It is to be found in the vicinity of Dorking, in Surrey. "_Bulimus_ is a large genus. A common small species is the _Gaudaloupe Bulimus_. "The largest land-shells are found in the genus _Achatina_. The greater number are African. "_Succinea_ contains a few species. One of them, _S. amphibia_, is common near fresh-water. "_Auricula_ has some resemblance to a _Voluta_. The aperture is longitudinal: the columella has one or more folds.