642 MOLLUSCA— CLASS IV.— GASTROPODA. 



single genua Cli'ilinia has a globose shell with wide aperture, a thickened 

 columella with one or two strong folds ; the exterior is ornamented with zig- 

 zag lines and spots of colour. 



The LiMNjEiD^ include our commonest pond-snails, Limncea and 

 Planorbis, as well as the little fresh-water limpet, Ancylus, with a few 

 other allied genera. A characteristic feature of their sheila is that their 

 surface looks as if it had been beaten with a hammer. 



The Limnseidse are amongst the most widely distributed of Mollusca, 

 being found from arctic to tropical climes, whilst Limncea can even live in 

 hot springs where the water attains a temperature of 104° F. The shells 

 of Limncea vary from very high to very short spired forms, but all have a 

 very large body-whorl. The genus Planorbis, or the flat-coil shells, are 

 really sinistral forms. Ancylus is found clinging to atones in swift-flowing 

 streams. 



The Physid^ are sinistral forms, and have very thin, globular, horn- 

 coloured, and glossy little shells. 



Sub-order 2. — Sttlommatophoiia. The Stylommatophora have, as a rule, 

 two pairs of tentacles, which can be withdrawn into the body by a process 

 of inversion, and carry their eyes on the summits of the upper pair. They 

 are all terrestrial forms and have an external spiral shell , except the slugs, 

 in which it is reduced to an internal shelly plate, or ia wanting altogether. 

 The slugs do not constitute a group by themselves, but belong to difierent 

 families. 



The land-snails occur in all climes and at all altitudes where life 13 

 possible, and include innumerable forms which have been variously 

 divided into a great number of familiea and aections ; the following, how- 

 ever, may be taken as constituting the principal groups : — 



The Onchidiid^33 are slug-like animals, entirely devoid of any shell, that 

 live close down to the margin of the sea. Only one pair of retractile 

 tentacles, bearing the eyes, is present. Certain species are also furnished 

 with eyes on the back. There is a British representative found locally on 

 the coasts of Cornwall and Devon. 



The VaginuliDjS! are shell-less slugs, unrepresented in Europe. Only 

 one genus, Vaginula, is distinguished. 



The ATHORACOPHOEiDiE, with a single genus, Aihoracophorus, have a small, 

 fiat, internal, shelly plate. The lower pair of tentacles is wanting. They are 

 found in New Zealand, New Caledonia, and the New Hebrides. 



The SucciNEiD^ include snails, in some of which the shell is external, 

 whilst in others it is internal. The lower pair of tentacles is either little 

 developed or wanting. The type genus Succinea, or the amber-snails, are 

 world-wide in their distribution, and have pretty little delicate, amber- ■ 

 culoured, glossy, spiral shells. Two or three species are British. Hyalimax 

 ia one of the slug-like members of the family found in the Maacareigne 

 and Nicobar Islands. 



The AcHAHKELLiD.Ts; occur in the Sandwich Is^lands. They are prettily 

 marked, and very variable little high-spired shells, that are sometimes 

 dextral and sometimes sinistral. 



The AcHATiNiDiE include at the same time some of the largest and of the 

 smallest land-snails. The type genus Achatina is confined to Africa and 

 its islands, although one species has been introduced into the Botanical 

 Gardens in India. It has a high-spired shell, usually with a large body- ' 



