The _Trochidæ_, and _Turbinidæ_ are two extensive families, the animals of which are very much alike, and mainly distinguished by the operculum, which in the former is horny, and shelly in the latter. The shells of these families are beautifully pearly within, and the external shelly coat is generally brightly coloured and highly ornamented. Several very pretty species are found on our own shores. The opercula of _Turbo petholatus_ (Fig. 6), from the Indian and Pacific Oceans, are frequently mounted in gold and silver as scarf-pins, ear-rings, &c. [Sidenote: Cases 18–20.] The _Nerites_ are mostly found in tropical countries, and, like the Winkles, are very strongly made, to resist the force of the breaking waves. The _Neritinas_ are partly found in the sea, and partly in fresh water, and are less solid shells. The third section of _Neritidæ_, the _Septariæ_, are shaped very much like Limpets, except that the apex is at one end instead of central. They are, however, very different animals, and furnished with a shelly operculum imbedded in the foot. Order 2.—PECTINIBRANCHIA. [Sidenote: Cases 22–94.] In most cases the molluscs of this order have an attached monopectinate gill and a single osphradium. A few are fresh or brackish water forms, but the majority are marine. [Sidenote: Case 22.] The “River-Snails” (_Viviparidæ_) might be termed freshwater Periwinkles, as the animals of both are very similar. The true _Viviparæ_ are viviparous. They are rather sluggish, and found at the bottom of ponds and rivers feeding on decaying animal and vegetable matter. [Illustration: Fig. 7. The Common British River-Snail (_Vivipara vivipara_). _a_, head; _b_, tentacles; _c_, eyes; _d_, foot; _e_, operculum. ] [Sidenote: Cases 23–25.] The _Cyclophoridæ_ are land-shells, which, however, cannot properly be considered true lung-breathers like ordinary snails. They have not the closed lung-chamber of the Pulmonates, their eyes are placed at the base of the tentacles instead of at their tips, they have a long proboscis armed with a different rasping tongue (_radula_), a spiral operculum, and the sexes are distinct, whereas the true Snails are hermaphrodite. The operculated air-breathers have been divided into many sections, chiefly on account of differences in the apertures of the shells and in the opercula. They most abound in hot countries, but a few species are met with in temperate regions. [Sidenote: Cases 25–26.] The “Apple-Snails” (_Ampullariidæ_) live in the rivers and marshes of tropical regions, and, although represented by a large number of species, exhibit comparatively slight variations in form and colour. The animal has both a pectinated gill and a lung cavity, being thus enabled to breathe either water or air. [Illustration: Fig. 8. _Ampullaria canaliculata._ ] [Sidenote: Cases 27–28.]