11. The _Calyptraeidae_ comprise the "Bonnet-limpet," or _Pileopsis_,
and "Cup-and-saucer-limpets" (_Calyptraea_). They may be described
briefly as limpets with traces of a spire left. The genus _Phorus_,
however, is spiral, and resembles a _Trochus_. They have been called
"Carriers" from their strange habit of building any stray fragments of
shell or stone into their house, thus rendering themselves almost
indistinguishable from the ground on which they crawl.
12. The _Turbinidae_, 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 _Haliotidae_ offer in the representative genus _Haliotis_, or
the "Ear-shell," another familiar mantelpiece ornament.
14. The _Ianthinidae_, 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 _Fissurellidae_, or "Key-hole" and "Notched limpets," whose
name sufficiently describes them. To these succeed--
16. The _Neritidae_, 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 _Patellidae_, 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 _Dentaliadae_, 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 _Chitonidae_, 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. _Helicidae_, 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.
[Illustration: Fig. 13. _Clausilia biplicata._]
2. The _Limacidae_, or "slugs," follow next; of these only one, the
genus _Testacella_, has an external shell stuck on the end of its
tail; the rest have either a more or less imperfect shell concealed
underneath the mantle, or else none at all.