Eleventh Class.--_Conchifera._
Two Orders.--1st. _Bimusculosa_, two muscular impressions.
2nd. _Unimusculosa_, one muscular impression.
[Sidenote: CONCHIFERA, SHELL-BEARERS.]
"This class," observed Mr. Elliot, "contains all the bivalves of
Linnæus, and some genera taken from the univalves and multivalves.
"The animals of this class are _shell-bearers_ or _carriers_, they
remain constantly fixed in their habitations: the body is fastened to
the shell by one or two strong muscles: when the shell is vacant we
find the _cicatrix_. Refer to your explanation of terms, and you will
find the word.
"The body is soft, without joints, without head or eyes; it is wrapped
in a mantle or tunic. The mouth, always hidden in the tunic, is merely
an opening to admit food, without jaws or teeth. The shell is always
bivalve; the valves united by a hinge or a ligament; sometimes there
are accessory pieces to the valves.
[Sidenote: CONCHIFERA. UNIMUSCULOSA.]
"Some of the _Conchifera_ are furnished with a kind of foot, which
enables them to move with their shells, to draw out fibres by which
they fasten themselves to marine bodies. The muscles that fasten the
animals to their shells are thick and strong; their use is, to close
the valves by contracting; when the muscle is relaxed, the elastic
ligament is sufficient to open them. The _Conchifera_ are all aquatic;
some inhabit fresh water, the others dwell in the sea.
"The class contains nineteen families and two orders. The first order,
_Bimusculosa_, contains thirteen families. The first includes genera
that you will scarcely expect to find among the bivalve shells.
* * * * *
"1st Family, _Tubicolaria_, contains,
_Aspergillum_, Watering-pot _Serpula._
_Clavagella_ -------
_Firtuluna_
_Septaria_ _Serpula._
_Teredina_ Fossil genus.
_Teredo_ _Teredo._
"_Aspergillum Javanum_ is a rare and curious shell from the Indian
seas, (Plate 9.) The whole family is remarkable, and was
referred, you perceive, to a very different order. _Clavagella_
was till lately considered as existing only in a fossil state. The
researches of recent travellers have discovered _Clavagella_ at Port
Jackson, in Australia.[A] There is a specimen in the British Museum.
The valves are enclosed in the tube.
[A] See Penny Cyclopædia, article Clavagella.
"The valves of _Teredo_ are noticed as forming part of the Linnæan
generic character, you will recollect. Lamarck considers them as true
_Conchifera_. In many specimens of _Teredo_ the valves are wanting, and
the tube only remains.
* * * * *
[Sidenote: PHOLADARIA.]
"The family _Pholadaria_ contains,
_Pholas._ _Pholas_, stone-piercer.
_Gastrochæna_ _Pholas_ and _Mya_.
"Notwithstanding the accessory pieces of the hinge, _Pholas_ is placed
among bivalve shells, the essential character of which is to have two
valves united by a hinge. The _Pholas_ has a foot or strong muscle,
very thick and short. In the next genus, composed of _Pholas hians_ and
_Mya dubia_ there are no secondary valves.
"Allied to this family is _Xylophaga dorsalis_, a curious shell. One
specimen has been lately found at Gravesend, upon a stick.
* * * * *
[Sidenote: SOLENACEA. MYARIA. MACTRACEA.]
"_Solenacea_ includes
_Solen_ _Solen._
_Panopæa_ _Mya._
_Glycimeris_ _Mya._
"The _Solen_ is furnished with a muscle, called by some writers a
tongue. By the aid of this instrument they descend two feet deep in the
sand. The tongue is first projected from the shell, and cuts a hole.
It then assumes the form of a hook, and draws down the shell into the
hole. This operation is repeated until the shell disappears. _Panopæa_
is a large shell--it is in the Museum.
* * * * *
"4th Family, _Myaria_.
_Mya_ _Mya_, or gaper.
_Anatina_ _Mya._
"The animal of _Mya_ has also a foot: it buries itself in the sand. You
know the broad tooth of the _Mya_ genus. _Anatina_ has a tooth on each
valve.
* * * * *
"Second Section contains four families.
"_Mactracea_ has the following genera:
_Lutraria_ _Mactra._
_Mactra_ _Mactra._
_Crassatella_ _Mactra_.
_Erycina_ ----
_Ungulina_ ----
_Solenomya_ _Mya_.
_Amphidesma_ _Tellìna_.
"_Crassatella_ is a genus from the seas of New Holland. The shell
is very thick, with a brown epidermis. A fossil species is found at
Hordwell cliff. There are several species also found in the chalk.
_Mactra_, _Lutraria_, and _Erycina_ are found in a fossil state.
_Crassatella sulcata_ is common in London clay.
* * * * *
[Sidenote: CORBULA. LITHOPHAGA.]
"The family _Corbula_ contains two genera.
_Corbula_ ----
_Pandora_ _Tellìna_.
"_Corbula_ comes chiefly from the Asiatic Seas. There is _one_ species,
formerly _Mya inequivalvis_, from the British Ocean; fossil species
several. _Pandora rostrata_ is British, and is said to be met with at
Weymouth. It is a pretty shell. The ligament of these is internal.
* * * * *
"_Lithophaga_ includes
_Saxicava_ _Mytilus_.
_Petricola_
_Venerupis_, or Venus of the rocks.
[Sidenote: VENERUPIS PERFORANS. PSAMMOBIA.]
"These genera consist mostly of small shells, inhabiting stones, into
which they bore holes. _S. rugosa_ is British. _Venerupis perforans_
is found on our coasts in stones. The valves of these shells have no
accessory pieces like _Pholas_.
* * * * *
"_Nymphacea_ is the next family, containing, in the first section,
_Sanguinolaria_ _Solen_.
_Psammobia_ _Tellina_.
_Psammotæa_ ----
"In the genus _Psammobia_ we find our _Tellina Feroensis_. (Plate
4.) The shells of this and the preceding genus resemble the solens
in a trifling degree, being a little open at the sides. In form they
are near _Tellina_, but have not the fold on the anterior valve, but an
angle on _each_ valve. The ligament is exterior.
* * * * *
"In the second section are--
_Tellina_ _Tellina_.
_Tellinides_ _Tellina_.
_Corbis_ _Venus_.
_Lucina_ _Venus_ and _Tellina_.
_Donax_ _Donax_.
_Capsa_ _Donax_.
_Crassina_ _Venus_.
[Sidenote: CORBIS. LUCINA. CAPSA. CYCLAS.]
"There is but one species of _Tellinides_ from the island of Timor. The
genus _Corbis_ is fossil, with one exception, _Corbis fimbriata_, from
the Indian Ocean. _Lucina_ is a pretty genus of shells. _L. carnaria_
is frequently found in collections. The interior of the valves is of a
deep red colour: the muscular impressions are very distant from each
other; one is greatly lengthened out; the valves delicately striated.
_Capsa_ is taken, you perceive, from _Donax_.
"_Tellina_ is found fossil on the borders of the Red Sea, also in the
county of York. Of _Donax_ and _Mactra_ the fossil species are few.
"In the third section of this order we find six families. 1st.
_Conchæ_, which are of two kinds, fluviatic, living in fresh-waters;
and marine, or living in the sea. Of the first are,
_Cyclas_, taken from _Tellina_.
_Cyrene_, partly from _Tellina_ and _Venus_.
_Galathea_, _Venus paradoxa_, (one species.)
"_Cyclas rivicola_ (Plate 9.) will give you an idea of this
genus: it is _Tellina cornea_ of Linnæus. The species are very common
in lakes, rivers, and ponds: it abounds in river-sand, from which you
may often procure perfect specimens. Lamarck observes that it is rare
in France; but appears common in the Thames.
[Illustration: Plate 9.]
[Sidenote: POTAMOPHILA. ASTARTE. PULLASTRA.]
"_Cyrene_ is a foreign genus.
"In the _Conchæ marìnæ_ the genera are very numerous. They are all
assembled under the _Venus_ of Linnæus. Lamarck reduced the genus; but
it has been yet further divided by later writers.
_Cyprina_, }
_Cytherea_, } From _Venus_, Lamarck's genera.
_Venus_, }
But _Pullastra_,
_Astarte_,
_Venerupes_,
and _Potamophila_ have been since withdrawn from the original genus.
* * * * *
"_Pullastra_ was the name of a species, and includes _V. pullastra_,
_V. papilionacea_, _V. decussata_, _V. litterata_, _V. virginea_.
"_Astarte_ includes some British species, _V. Scotica_, &c.
[Sidenote: VENUS. CYTHEREA. ISOCARDIA.]
"_Potamophila_ is a scarce river-shell from Ceylon. Some species
have also been brought from Congo by African travellers. The form
is triangular, very thick, covered with an olive-green epidermis.
Lamarck's two genera have been still further reduced; but I shall refer
you to the Museum for their new names. Observe, in _Venus_ there are
three cardinal teeth, close together, on each valve, with divergent
lateral teeth. _V. lamellata_ is rare and beautiful, from the seas of
New Holland. There are many species of _Venus_ in a fossil state. In
_Cytherea_ we find four cardinal teeth on the _right_ valve, three of
them near together, the fourth quite apart. The _left_ valve has three
cardinal teeth. _C. Dionè_, the thorny Venus, is a pretty shell with
spines. You may easily procure it.
"_Astarte_ has some fossil species in the crag and green sand: _A.
obliquata_ is one species.
"_Venericardia_ is wholly a fossil genus: one species is found in the
crag, _V. senilis_.
* * * * *
"The family _Cardiacea_ contains
_Cardium_ _Cockle_
_Cardita_ _Chama_ (some species.)
_Cypricardia_ ------
_Hiatella_ _Mya_.
_Isocardia_ _Chama_.
"_Isocardia cor_ is British. (Plate 9.) There is a beautiful
species, _Isocardia moltkiana_ from the East Indies, which is much
valued by collectors.
* * * * *
"In the family _Arcacea_ we find,
_Cucullea_ _Arca_.
_Arca_ _Arca_, ark-shell.
_Pectunculus_ _Arca_.
_Nucula_ _Arca_.
[Sidenote: ARCA. PECTUNCULUS. NUCULA. NAYADA.]
"The hinge of _Arca_ in this arrangement is always _straight_,
furnished with a number of teeth; the ligament is external. The shells
are open at one end, for the animal throws out at the aperture a number
of threads, by which it fastens itself to the rocks. The species are
thirty-seven, and also several fossil.
"The orbicular form of _Pectunculus_, and its arched hinge, distinguish
this genus from the preceding one. They are allied to the Pectens by
their form, and their crenulated internal margin.
"The hinge of _Nucula_ is set with little teeth on each side, like a
comb. It is pearly within, and sometimes small pearls are found in the
shell. _Pectunculus costatus_ is found in London clay.
"_Trigoniana_ is a small family containing _Trigonia_ and _Castalia_.
The first is a fossil genus chiefly. Some species are found in the
Portland stone, or oölite beds.
"The next family contains the _Nayada_, chiefly composed of fluviatic,
or fresh-water shells. They are covered with an olive-brown epidermis,
which is constantly found eroded, or destroyed at the beaks. The
muscular impressions are lateral and much separated; one of them is
formed of two or three distinct irregular impressions.
_Unio_, taken chiefly from _Mya_.
_Hyria_ _Mya_.
_Anodon_ _Mytilus_.
_Iridina_ Very rare genus.
[Sidenote: UNIO. ANODON. DICERAS.]
"_Unio_ has two teeth on each valve; one is cardinal, the other
lengthened out. The ligament is exterior--the shell pearly. _Unio
pictorum_ is common in rivers. The shell is used to hold small masses
of gold or silver for artists, under the name of _shell-gold_.
"_Anodon_ is also to be met with in our rivers.
"_A. anatina_ is eaten by ducks and crows. The latter, when the shell
proves too hard to penetrate, mount with it into the air, and letting
it fall, pick out the fish from the broken shell.
* * * * *
"_Chamacea_ has only three genera.
_Diceras._
_Chama_ _Chama_.
_Etheria_, a rare genus, from the Indies and Madagascar.
* * * * *
"_Diceras_ is a fossil genus--only two species known according to
Lamarck.
[Sidenote: CHAMA.]
"Linnæus had assembled in his genus, _Chama_, shells with equal and
with unequal valves, shells fixed to other marine bodies, with those
that are free; some with _one_, others with _two_ muscular impressions.
In the present genus, _Chama_, the shells are irregular, thick, scaly,
or spinous. The hinge has one thick tooth, often notched: the beaks are
bent inwards. They are found in the Indian, American, and Mediterranean
seas. There are several fossil species.
"The first order, _Bimusculosa_, is finished. In our next lesson we
shall proceed to the families and genera contained in the second,
_Unimusculosa_."