The “Violet Snails” (_Ianthinidæ_) are found floating about in every ocean, excepting in cold regions, with the spire of the shell downwards, and the bottom, being more exposed to the action of light, is more deeply tinted than the upper part. They feed upon Jelly-fish, and construct a gelatinous raft, filled with air-bubbles, beneath which the females attach their eggs. [Illustration: Fig. 10. _Cypræa leucodon_. Case 32. ] [Sidenote: Cases 38–41.] The _Melaniidæ_ are freshwater Snails which abound in most tropical and subtropical countries; about 1000 species are known. They are mostly of dark colours, and are fond of muddy places. [Sidenote: Cases 42–43.] The _Cerithiidæ_ are chiefly marine forms, some, however, entering brackish water. About five hundred fossil species have been described, some of them gigantic in comparison with any now living, of which more than two hundred are known. [Sidenote: Case 44.] The _Scala scalaris_ was formerly considered a great rarity, as much as £40 having been given for a single specimen, which might now be purchased for as many pence. [Illustration: Fig. 11. _Scala scalaris._ Case 44. ] [Sidenote: Cases 46–47.] The “Worm-shells” (_Vermetidæ_) are a very peculiar family. Their shells can scarcely be distinguished from the shelly tubes which are formed by certain species of marine worms, _Serpula_, &c. They are free and spiral in early life, but afterwards become distorted and generally attached to rocks, stones, &c. A foot for walking purposes therefore would be of no use; consequently it is more or less obsolete, serving only as a support to the operculum. [Sidenote: Case 48.] The “Screw-shells” (_Turritellidæ_) have elongate tapering shells; about 100 recent and 200 fossil species are known. One species only (_Turritella communis_) is now found living on the British coasts. [Sidenote: Case 49.] The _Xenophoridæ_ have the singular habit of cementing to the exterior of their shell, stones, pieces of coral, and fragments of other shells; hence they have been called “Carrier-shells,” and, according to the kind of material chosen, have been named “Conchologists” and “Mineralogists.” Beyond acting as a disguise, and consequently as a protection, there does not appear to be any special utility in thus adding to the weight of their own shells. The animals do not glide like most other molluscs, but scramble along like the Strombs, the form of their foot being small, divided into a front, expanded, and a hind, tapering portion admirably adapted to the nature of the ground on which they live, which usually consists of broken and dead shells. [Sidenote: Cases 49–52.] The “Wing-shells” (_Strombidæ_) are the largest of the Gastropods with a proboscis or non-retractile snout. They do not crawl like most other Gastropods, but progress by a sort of hopping movement. They act as scavengers, feeding on decomposing animal matter.