Second Division.--_Bivalves_: 14 genera.
"I fear," said Charles, "that this new division will be rather
difficult, for my father tells me that we must pay particular attention
to the _hinges_ of bivalve shells."
"Then _apply_ yourself to the study of hinges, Charles, and your
difficulties will chiefly disappear," answered Mr. Elliot.
[Sidenote: MYA.]
"The hinge of _Mya_, the first on the list, is easily known. The
generic characters are, shell gaping at one end, hinge mostly with one
thick spreading tooth, not inserted into the opposite valve. The _Mya_
race burrow in the sand. Here is _Mya arenaria_, a large thick shell,
frequent on the shores of Kent: the large tooth is sufficiently plain
in _this_ species. _Mya truncata_ (Plate 4) is as common, and
the curious membranous case, which you will find attached to one end of
the shell, is a guide to the species. Both these species are without
colour, and have little to attract in their outward appearance. The
genus, however, according to Linnæus, varies exceedingly, and contains
forty-one species.
[Sidenote: SOLEN. TELLINA.]
"_Solen._ Shell bivalve, open at both ends, tooth of the hinge
subulate, or awl-shaped, reflex, often double.
"In this genus, the great length, in comparison with the breadth of
the shells in many of the species, is remarkable: some are exceedingly
brittle. Our example, _Solen siliqua_ (Plate 1) is a British
species. The hinge is not in the centre of the shell, but nearer to
one end of it. Some are shaped like the handle of a knife or a razor,
others are bent resembling the blade of a scimitar. The _Solen_ lives
in the sands of the sea-shore, often burying itself two feet deep, and
retaining its shell in a vertical position: thirty-five species.
"The genus _Tellìna_ is remarkable for the beauty of the shells, and,
according to the arrangement of Linnæus, contains ninety-seven species.
The exterior is sometimes marked with radiations: the surface of some
shells is very finely polished, while in others it is covered with
striæ and undulations. The species that you have placed before me,
Lucy, is _Tellìna Feroensis_; the shell is finely striated, and has
also radiations. (Plate 4.)
"The generic characters are chiefly these: shell compressed towards
the anterior slope, teeth of the hinge mostly three, the lateral ones
smooth, in one valve. Two or three small species are common on our
coasts. I should also observe that there is a convex fold on one valve
and a concave fold upon the other. Many of the _Tellìna_ genus are
found buried in the sea-sands.
[Sidenote: CARDIUM. MACTRA.]
"_Cardium._ Generic character: shell equivalve, convex, ribbed,
striated, or grooved, the margin toothed: hinge with two teeth near the
beak, and a lateral one on each side: fifty-four species.
"Observe how the beaks of this common _Cardium_, cockle, turn inwards,
and the bosses project. Another striking character is the ribs, that
are generally longitudinal, and not concentric or transverse, as in
_Tellìna_, and, as you will see, in _Venus_. _C. aculeatum_ has small
spines on the valves; _C. costatum_, the ribbed cockle, is one of the
finest species of this genus, and _C. cardissa_ is a beautiful shell.
(Plate 4.) The common cockle is _Cardium edule_.
_Mactra._ Generic character: shell bivalve, unequal sided, middle tooth
of the hinge complicated, with a small hollow on each side, and lateral
side-teeth: thirty-seven species.
"The shells of this genus are usually thin, brittle, and remarkably
light. _Mactra stultorum_ is a common species. (Plate 4.)
[Sidenote: DONAX. VENUS.]
"_Donax._ Margin of the shell often crenulate, the anterior slope very
obtuse; hinge with two cardinal teeth, and one lateral tooth.
"The most striking characteristic of _Donax_ is the broad, thick
extremity of one end, gradually lessening towards the other. A rich
purple tint is very frequent in these shells. _Donax denticulatus_ and
_trunculus_ are common British examples. (Plate 4.) You must
remark the ligament of _Donax_, which is exterior.
"Our next genus ranks highest for beauty among the bivalves, and takes
its name from the goddess _Venus_. The species amount to one hundred
and sixteen in the Linnæan system, but other authors have formed
several new genera.
"Shell bivalve, having the lips incumbent on the anterior margin; hinge
with three teeth, all approximate, the lateral ones diverging at the
lip.
"I am afraid," said Lucy, "that we shall find this genus very
difficult: I wish you would tell us the new genera that have been
formed out of it."
"Learn first to know the general appearance of _Venus_, and remark
especially the _teeth_. You may also bear in mind that the _beaks_ are
almost always turned _to_ the posterior slope, and _from_ the ligament.
The area and areola are also very conspicuous: the area is generally
large, and differently coloured to the disk. _Venus Paphia_ is pretty.
(Plate 4.) The spinous species, _V. Diòne_, is more beautiful,
and is the only shell of the genus that has spines. The brown Venus,
_V. chionè_, is very smooth and polished; both species are frequent in
collections. The British shells of this genus are neither numerous nor
very beautiful.
[Illustration: Plate 4.]
[Sidenote: SPONDYLUS.]
"_Spondylus._ Valves unequal, rough; hinge with two recurved teeth,
with a hollow between them; shell sometimes eared. (See Plates
3 and 5.)
"I think," said Lucy, "that the English name, _thorny-oyster_, is not
very suitable: it is more like a scallop; but it differs from both in
having two strong teeth in the hinge, and I observed this morning that
neither the oyster nor the scallop have any hinge."
"So that was the object you had in view," said Charles, "when you
were so quietly handling those shells in the kitchen: I confess I
could hardly help laughing; and now my father will say that _you_ are
'_Eyes_,' and I the '_No Eyes_,' of 'Evenings at Home.'"
"Perhaps I might have made the observation," replied Mr. Elliot; "but
you have reproved yourself, which is far better.
"_Spondylus_ can scarcely be mistaken from any other bivalve shell. The
species _gæderopus_ is remarkable for its projecting beak; the surface
is rough, with either tubercles or spines. Some authors reckon only
four species, others thirteen. The _Spondyli_ are frequently found
attached to rocks at some depth in the ocean. The animal is eaten on
the coasts of the Mediterranean. We have no British _Spondylus_.
[Sidenote: CHAMA. ARCA. OSTREA.]
"_Chama._ Shell thick; hinge with a thick tooth, sometimes crenate,
obliquely inserted into a corresponding channel. (Plate 5.)
The shells of this genus vary greatly, which you will perceive upon
comparing _C. gigas_ and _C. cor_. (See Plate 9, _Isocardia
cor_.) The _Chama_ genus is usually ribbed, foliated, or scaly. _C.
Lazarus_ is a beautiful species: _C. cor_ is a British species, and the
only one. The whole number is twenty-five.
"Here is _Noah's-ark_, an example of the genus _Arca_, and is found on
our own coasts. The long hinge beset with sharp teeth, inserted into
each other, renders the genus sufficiently marked; but in some species
the hinge is curved. The form varies exceedingly. The number of species
is forty-five. (Plate 5.)
"_Ostrea._ In this well-known genus we lose sight of the _toothed_
hinge. Take that _Pecten_, or scallop, which belongs to one division of
_Ostrea_ in this system, and tell me what holds the valves together.
Charles is silent; what says Lucy?"
"Here are the remains of the same kind of substance which we saw in
_Donax_ and in _Venus_. I think it is called the _ligament_."
[Illustration: Plate 5.]
"Very well remembered," continued her father. "The generic character
of this very large portion of bivalves is, shell bivalve, usually with
unequal valves: hinge without teeth, having a hollow cavity or sinus,
and sometimes grooved. Here is a young common oyster, and, according
to the rule of our present system, this shell, _Ostrea isognomon_,
is of the same genus. (Plate 5.) The number of species is
eighty-four, of which thirteen are British. The old shells of common
oysters are often covered with _Serpula_, _Lepas_, and _Anomia_, and
some kinds of corallines.
[Sidenote: ANOMIA. MYTILUS.]
"The next genus, _Anomia_, is remarkable for the thin, delicate, and
almost transparent appearance of the shells. The valves are unequal,
and frequently perforated near the apex; hinge toothless; in the flat
valve, two bony rays.
"_Anomia ephippium_ has a large perforation, through which the animal
passes a ligament, and attaches itself to other substances. These
shells are often to be found on oysters. (Plate 5.) Species
thirty-two.
"_Mytilus._ The principal characters are, shell bivalve, rough, often
affixed by a thick byssus, or beard; hinge without teeth, with a hollow
line extending lengthways. (Plate 3.)
"The common muscle, _Mytilus edulis_, must be well-known to you,
and also the fine polish that the shells will take when cleared of
the rough exterior by artificial means, _Mytilus barbatus_ is not
unfrequent on our shores; the colour is brown, and the shell is shaggy.
Number of species, forty-nine.
[Sidenote: PINNA.]
"Our last bivalve genus is _Pinna_. The generic characters--shell
bivalve, brittle, erect, gaping at one end, throwing out a byssus;
hinge without teeth. (Plate 6.)
"The _Pinna_ race are found plentifully in the Mediterranean, the
Indian, American, and Atlantic oceans: the British seas afford three
species. The genus is noted for producing a fine byssus, that is
manufactured in Italy into various articles, as gloves. The animal is
sometimes used as an article of food.
"An ancient writer asserts that the _Pinna_ is attended by a crab,
that finds a habitation in its shell, and repays the favour by giving
notice, by a gentle nip, when a fish comes within reach; the _Pinna_
opens the valves of the shell, and secures the prey, which serves
for the food of both. Now, Charles, you know the whole sense of the
quotation--
"'The anchor'd pinna and his cancer friend.'"