12. The Turbinidæ, or "Top-shells," are next in order, and of these the great Turbo marmoreus is a well-known example, being prepared as an ornament for the whatnot or mantelpiece by removing the external layer of the shell in order to display the brilliant pearly nacre below. These mollusca close their mouths with a horny operculum, coated on its exterior by a thick layer of porcelain-like shelly matter. With them are classed the familiar Trochus and other closely allied genera.
13. The Haliotidæ offer in the representative genus Haliotis, or the "Ear-shell," another familiar mantelpiece ornament.
14. The Ianthinidæ, or "Violet-snails," that float about in mid-Atlantic upon the gulf-weed, and at certain seasons secrete a curious float or raft, to which their eggs are attached, are next in order, and are followed by—
15. The Fissurellidæ, or "Key-hole" and "Notched limpets," whose name sufficiently describes them. To these succeed—
16. The Neritidæ, an unmistakable group of globular shells, having next to no spire and a very glossy exterior, generally ornamented with a great variety of spots and bands.
17. The Patellidæ, or true Limpets, are well known to every sea-side visitor: large species, as much as two inches across, are to be found on the coast of Devon, but these are pigmies compared with a South American variety which attains a foot in diameter.
18. The Dentaliadæ, represented by the genus Dentalium, or "Tooth-shell," are simply slightly curved tubes, open at both ends and tapering from the mouth downwards, and cannot be mistaken.
19. Lastly, we have the Chitonidæ, whose single genus Chiton possesses shells differing from all other mollusca in being composed of eight plates overlapping each other, and in appearance reminding one of the wood-louse. This animal is not only like the limpet in form but also in habits, being found adhering to the rocks and stones at low-water.
Order II.—Pulmonifera. Contains the air-breathing Gasteropods, and to it consequently belong all the terrestrial mollusca, though some few aquatic genera are also included. The members of this order have an air-chamber instead of gills, and are divided into two groups, (a) those without an operculum, and (b) those having an operculum. Foremost in the first group stands the great family—
1. Helicidæ, named after its chief representative, the genus Helix. It also includes the "Glass-shell" (Vitrina), the "Amber-shell" (Succinea), and such genera as Bulimus, Achatina, Pupa, Clausilia ([Fig. 13]), etc., which differ from the typical Helix in appearance, possessing as they do comparatively high spires.