"The top-shell, _Trochus_, is univalve, conic, spiral; the aperture either angular or rounded; columella oblique: some of the apertures have a tooth-like projection. (Plate 7.) Species, one hundred and thirty. Several kinds occur in Britain. New Zealand, Friendly Isles, Red Sea, and most other seas, afford the various species. Two of this genus have the power of collecting parts of shells and other testaceous substances, which adhere strongly to the whorls of the shell: it is called the Conchologist. The other, named Mineralogist, is loaded with stones, pebbles, ores, &c. When heavily laden they are considered rarities. [Sidenote: TROCHUS. TURBO. HELIX.] "There is a great similarity between the genera _Turbo_ and _Trochus_. You must observe the generic distinction carefully. Shell univalve, spiral; aperture contracted, orbicular, entire. The one hundred and sixty-seven species have been much divided by other writers. The golden-mouthed _Turbo_ is a very fine shell. This genus also contains the common periwincle, an inhabitant of most European shores. Sailors report that if the animal is seen creeping high up the rocks, it foretells stormy weather. _Turbo muricatus_ is a pretty shell." (Plate 7.) "Now we can tell the next genus," said Lucy. "_Helix_, snail. But what a number of different shapes, father, those shells have! they are not all snails, I should think, that you have placed on the table." [Illustration: Plate 7.] "According to Linnæus they are all of the genus _Helix_, which contains two hundred and sixty-seven species. Many kinds are land-shells; others live in fresh water; few inhabit the sea. "Shell univalve, spiral, brittle; aperture contracted, semi-lunar, or roundish. The common snail is well-known to most persons. _Helix nemoralis_, the wood-snail, is very pretty; sometimes it is pink, with brown bands, or plain yellow, or yellowy banded with brown. (Plate 7.) The greater part of this genus consists of shells remarkable for their thin, brittle, and semi-transparent substances. [Sidenote: NERITA. HALIOTIS.] "The _Nerìta_ genus is very pretty: (Plate 7.) the texture of the shell is in general much thicker than that of _Helix_. The shell is spiral, gibbous, pillar-lip transversely truncated, flattish. Seventy-six species. _Nerìta polìta_ is a handsome species: those most valued are from the South Sea. "We have now lost sight of the pillar-lip, and in the genus _Haliòtis_ we find a flat, ear-shaped shell, the spire nearly hidden, the disk perforated lengthways with pores. Species twenty-one. The animals that inhabit the ear-shells fasten themselves so firmly to the surfaces of rocks, that much force is needful to disengage them: during the fine nights of summer, the animal feeds on the herbage that grows on the sea-shore. The sea-ear from New Zealand, and that from California, are superb shells of considerable size. The British species, _Haliòtis tuberculàta_, is not uncommon. (Plate 8.)