Third Division.--_Univalves._
"Father," said Lucy, the next time they met to pursue their study, "I
think we have made ourselves familiar with the various hinges of the
bivalve shells, which are becoming favourites with us; but from the
variety of fine specimens which you have on your table, I see that we
shall be much gratified in examining the univalve division."
[Sidenote: ARGONAUTA.]
"My first genus is very beautiful," replied Mr. Elliot; "it is
_Argonauta_, or paper-sailor. The shell is univalve, involute,
unilocular, or without chambers: the aperture cordate. (Plate
6.) These shells are spiral, and remarkably brittle. The argonauts
are supposed to be the shells that taught mankind the use of sails in
the earliest ages of society. In calm weather the animal rises with its
shell to the surface of the water, and spreads its arms over the edge;
these arms answer the purpose of oars. It then spreads a membrane for
a sail, which can be turned in any direction, and is impelled forwards
by the breeze: two other arms serve as rudders to direct the course.
The animal first raises itself to the surface of the sea by ejecting
a quantity of water; if danger occurs, it absorbs water, and thus, by
making itself heavier, sinks to the bottom. The species are few in
number.
[Sidenote: NAUTILUS. CONUS.]
"_Nautilus_, pearly sailor, has several characteristics of _Argonauta_;
but the former is concamerate, the latter without chambers in the
shell. The generic characters of _Nautilus_ are, shell univalve,
divided into several compartments, communicating with each other by an
aperture. _Nautilus pompilius_ is often cut through, or bisected, to
display the chambers of the shell. In the East, the shells are formed
into drinking-cups. Sometimes the outer coat of the shell is removed,
and the pearly surface finely carved. This genus, according to Linnæus,
consists of fifty-eight species, some of which are fossil. (Plate
6.)
"In the following genera we must pay particular attention to the
_aperture_ of the shell, which is a generic distinction in most
univalves.
[Illustration: Plate 6.]
"The first is _Conus_, a large and beautiful genus, including many rare
and valuable species. Shell univalve, turbinate, aperture effuse, or
having the lips separated by a sinus, linear, without teeth, pillar
smooth. In their natural state the shells are usually covered with an
epidermis; but will bear a brilliant polish. _C. textilis_, cloth of
gold, is valuable. _C. generalis_ is sometimes sold for twenty guineas.
The example on the table is _C. Ebræus_, or Hebrew cone. (Plate
6.) Species one hundred and fifty-five; not one British. The
greater number are brought from the Indian Ocean; some from the seas of
Africa and from the South Sea.
[Sidenote: CYPRÆA. BULLA.]
"The shells of the genus _Cypræa_, cowry, are general favourites: the
species are fifty-eight; one British, _C. pediculus_. _C. moneta_
(Plate 1) is very common. The generic characters are, shell
univalve, involute, obtuse, smooth; aperture linear, the whole length
of the shell; effuse at both ends, toothed on each side.
"Look carefully at those three shells: do you perceive much resemblance
between them? 'Not much, if any,' you reply, yet they are all of
the genus _Bulla_. Here is _B. lignaria_, _B. terebellum_, (see
Frontispiece,) and _B. naucum_. (Plate 6.) There are
other forms, as the _B. ovum_, _B. volva_, the first somewhat like a
cowry; but it is toothed only on one side of the aperture; the second
has two long beaks.
"This genus is confessedly ill-determined. _B. naucum_ and _B. ampulla_
are examples of the common characters of the genus. The species are
sixty-one.
[Sidenote: VOLUTA. BUCCINUM.]
"_Voluta_ is also a large genus, containing one hundred and eighty-six
species. Shell univalve, aperture without a beak, and somewhat effuse;
columella _plaited_. This latter character we meet with for the first
time. Here is the common _Voluta_. (Plate 6.) The genus has
been much diminished by forming other very striking genera out of
it, as I shall soon show you, under the names of _Mitres_, _Olives_,
_Gondolas_, &c. _Voluta musica_, the music-shell, is remarkable, and
not at all rare.
"_Buccinum_ is another large genus. The shell univalve, spiral,
gibbous; aperture ovate, ending in a canal turning to the _right_, with
a short beak; pillar-lip expanded. Species, one hundred and seventy-two.
"You must recollect that when the apex of the shell is turned
_downwards_ the canal turns to the right, when it is turned _upwards_
the canal will be to the left hand. My example is _Buccinum
reticulatum_, a very common species upon our own shores." (Plate
7.)
"Is not this genus reduced by other authors?' asked Charles.
"Greatly," replied Mr. Elliot: "you will meet with tuns, helmets,
harps, and needles. Species of the _Buccinum_ genus are found in the
African, American, Indian, European, and Southern oceans. Eighteen
occur upon our coasts.
[Sidenote: STROMBUS. MUREX.]
"_Strombus_ contains forty-four species. Shell univalve, spiral,
aperture much dilated, the lip expanding, ending in a canal inclining
towards the left.
"You must notice the sinus in the outer lip, near the base of the
shell. _Strombus gigas_, the West Indian conch, is very large. Some
species have the lip ending in claws. _Strombus pes-pelicani_, the
pelican's-foot, has four palmated claws: (Plate 7.) it is a
British shell. The city of Santa Cruz, in America, is paved with the
shells of _Strombus gigas_.
"The genus _Murex_ is both large and beautiful. Shell univalve, spiral;
aperture oval, ending in a straight canal.
"These shells are of very unequal form; their surfaces frequently
covered with spines, knobs, or foliations. Some are remarkable for
the great length of the beak, (Frontispiece,) such as the
woodcock, the snipe's-head, and Venus's-comb. The _Murex_ before you
is foliated. (Plate 7.) The species are one hundred and
seventy-one. Several are found on British coasts, but they are not
remarkable for beauty.
"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.)
Without a regular spire: 5 genera.
[Sidenote: PATELLA. DENTALIUM. SERPULA.]
"You are well acquainted with _Patella_, the limpet: one species of
this genus is very common on the rocks by the sea-side. (Plate
1.) In the _Patella_ genus we lose sight of a spire; the shell is
nearly conic, and shaped like a basin. The species are very numerous,
exhibiting great variety of form; the number is two hundred and forty.
"The form of _Dentalium_ is easily known. The shell is univalve, nearly
straight, tubular, not chambered, and open at both ends.
"The species called elephant's-tooth is slightly curved, the colour
green, (Plate 2.) It is found in the European and Indian
seas. There are only twenty-two species. _Dentalium entails_, the
dog's-tooth, is very common.
"The _Serpula_ genus is remarkable. The shells are tubular, frequently
closed at one end. They are often found in clusters, adhering to rocks,
stones, fuci, shells, &c.
"There is _Serpula triquetra_ upon a _pecten_, (Plate 8.)
[Sidenote: TEREDO. SABELLA.]
"From the appearance of this piece of timber you may form some idea of
the devastation committed by the _Teredo_, or ship-worm. (Plate
8.) The shell is tubular and flexuose; two valves at each end, and
penetrating through wood. There are four species.
[Illustration: Plate 8.]
"_Sabella_ is the last genus, and a very remarkable one. The species
are twenty-five, several of which are British, (Plate 8.)
"Shell tubular, formed of sandy and calcareous particles, agglutinated,
and inserted in a membranous sheath. _Sabella Belgica_ is found in
Britain. _S. chrysodon_ is found buried in sea-sand, often several
inches long; it is covered with fragments of shells, and so brittle
that it is not easy to obtain a complete specimen.
"We have now finished our Linnæan genera, and here we must pause for
the present. If you wish for any assistance in your study of the
thirty-six examples that I have given you, I shall be ready to afford
you both any help that lies in my limited power."