All the foregoing processes, except the naming of your specimens, are
more or less mechanical, and are only the means to the end--a properly
arranged collection. For, however well a collection may be mounted, it
is practically useless if the different shells composing it be not
properly classified. By classification is meant the bringing together
those kinds that most resemble each other, first of all into large
groups having special characteristics in common, and then by
subdividing these into other smaller groups, and so on. Thus the
animal kingdom is divided, first of all, into _Sub-kingdoms_, then
each _Sub-kingdom_ into so many _Classes_ containing those which have
further characteristics in common, the _Classes_ into _Orders_, the
_Orders_ into _Families_, the _Families_ into _Genera_, and these
again into species or kinds.
The Mollusca, or soft-bodied animals, of whose protecting shells your
collection consists, form a sub-kingdom, and are subdivided into four
classes:--
1. Cephalopoda.
2. Gasteropoda.
3. Pteropoda.
4. Lamellibranchiata (or Conchifera).
And these again into Families, Genera, and Species.
The space at our disposal being limited, it is impossible to do more
than furnish some general outlines of the different forms. For further
details it will be necessary to refer to one of the larger works, a
list of which will be found on the last page.
[Illustration: Fig. 4. _Argonauta Argo._]
[Illustration: Fig. 5. "Bone" of _Sepia officinalis_.]
CLASS I.--The CEPHALOPODA (Head-footed) contains those mollusca that,
like the common Octopus, have a number of feet (or arms) set round
the mouth, and is divided into those having two gills. (Order I.
Dibranchiata); and those with four (Order II. Tetrabranchiata). Order
I. is again divided into: (_a._) Those with _eight_ feet like the
Argonaut (or Paper-nautilus, Fig. 4), which fable has so long endowed
with the power of sailing on the surface of the ocean, (it is even
represented in one book as propelling itself through the air!) and the
common Octopus. (_b._) Those with _ten_ feet, such as the Loligo (or
Squid, Fig. 6), whose delicate internal shell so much resembles a pen
in shape; the Cuttle-fish (Sepia, Figs. 5 & 7), whose so-called
"bone" (once largely used as an ink eraser) is frequently found on our
southern coasts; and the pretty little _Spirula_ (Fig. 8).
[Illustration: Fig. 6. _Loligo vulgaris_, and "Pen."]
[Illustration: Fig. 7. _Sepia officinalis._]
The only representative of the four-gilled order now living is the
well-known Pearly Nautilus; but in former times the Tetrabranchiata
were extremely numerous, especially the _Ammonites_.
[Illustration: Fig. 8. _Spirula_.]
CLASS II.--GASTEROPODA (Belly-footed) comprises those mollusca which,
like the common snail, creep on the under-surface of the body, and
with one exception (_Chiton_, Fig. 20) their shells are univalve
(_i.e._, composed of one piece). But before we go further, it may be
well to point out the names given to different parts of a univalve
shell. The aperture whence the animal issues is called the _mouth_,
and its outer edge the _lip_; each turn of the shell is a _whorl_; the
last and biggest, the _body-whorl_, the whorls, from the point at the
top, or _apex_, down to the mouth form the _spire_; and the line where
the whorls join each other is called the _suture_. The axis of the
shell around which the whorls are coiled is sometimes open or hollow,
and the shell is then said to be _umbilicated_ (as in Fig. 3_b_); when
closely coiled, a pillar of shell, or _columella_, is left (as in Fig.
9). Sometimes the corner of the mouth farthest from the spire and
next the columella, is produced into a channel, the _anterior canal_
(as in Fig. 9); whilst where the mouth meets the base of the spire
there may be a kind of notch which is termed the _posterior canal_.
Most Gasteropods are _dextral_, that is to say, the mouth is to the
right of the axis as you look at it; a few, however, are _sinistral_,
or wound to the left (like _Physa_); whilst reversed varieties of both
kinds are met with.
Gasteropods of the first order have comb-like gills placed in advance
of the heart, and are hence termed PROSOBRANCHIATA. They are divided
into two groups: (_a_) _Siphonostomata_ (Tube-mouthed), in which the
animal has a long proboscis, and a tube, or siphon, from the
breathing-chamber that passes along the anterior canal of the shell,
which in this group is well developed. They have a horny operculum, or
lid, with which to close the aperture. (_b_) _Holostomata_ (or
Whole-mouthed). In these the siphon is not so produced, and does not
want to be protected; accordingly the mouth of the shell is _entire_,
_i.e._ has no canal. The operculum is horny or shelly. The former
(group _a_) includes several families:
1. _Strombidae_, comprising shells, like the huge _Strombus_, or
"Fountain-shell," which is so often used to adorn the mantelpiece or
rockery, and from which cameos are cut.
2. The _Muricidae_, of which the _Murex_ (an extraordinary form of this
is the "Venus' comb," _Murex tenuispina_, Fig. 9), the Mitre-shells,
and the Red-Whelks (_Fusus_) are examples.
[Illustration: Fig. 9. _Murex tenuispina._]
3. The _Buccinidae_, taking its name from its type, the Common Whelk
(_Buccinum undatum_), and including such other forms as the Dog-Whelk
(_Nassa_), the _Purpura_, the strange _Magilus_, and the lovely
Harp-Shells and Olives (Fig. 10).
[Illustration: Fig. 10. _Oliva tessellata._]
4. The _Cassididae_, or "Helmet-Shells." _Cassis rufa_, from West
Africa, is noted as the best species of shell for cameo engraving;
with it are classed the "Tun" (_Dolium_) and the great "Triton"
(_Triton tritonis_), such as the sea-gods of mythology are represented
blowing into by way of trumpet, and which are used by the Polynesian
Islanders to this day instead of horns.
5. The _Conidae_, whose type, the "Cone-shell" (Fig. 11), is at once
distinctive and handsome, but which in the living state is covered by
a dull yellowish-brown periostracum that has to be carefully removed
before the full beauties of the shell are displayed.
[Illustration: Fig. 11. Conus vermiculatus.]