A Conchological Manual by George Brettingham Sowerby (early reader books .txt) 📖
- Author: George Brettingham Sowerby
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substances, such as Spondyli, Oysters, &c.; and the degree of irregularity
will depend upon the extent of surface involved in the attachment. In the
case of fixed shells, the attached valve is usually termed the under valve,
and the other which moves freely upon the hinge, is termed the upper valve.
_Form and Proportions._
Bivalves are said to be _equivalve_ when the two valves correspond in
extent, breadth, and thickness; and of course _inequivalve_ in the contrary
case. They are _equilateral_ when a line drawn from the umbones to the
ventral margin would divide the shell into two nearly equal parts; and of
course _inequilateral_ in the opposite case, which occurs in the great
majority of instances.
A Bivalve is said to be _compressed_, when the distance is small from the
most prominent part of one valve to that of the other. It is _cylindrical_
when lengthened, and more or less rounded in its breadth, as in Lithodomus
(fig. 161 in the plates). It is _cordiform_ when the shape presents a
resemblance to an imaginary heart, as in Cardium cardissa (fig. 122 in the
plates), and in the Isocardia (fig. 126 in the plates). It is _linguiform_
when it resembles a tongue in shape, as in Vulsella (fig. 185 in the
plates); _rostrated_ when it protrudes at either extremity, and terminates
in a kind of point, as in Sanguinolaria Diphos (fig. 99 in the plates);
_truncated_ when it ends in a square or angle, as if cut off; an example of
which may be seen in Solen (fig. 60 in the plates).
Other Bivalves are distinguished as being _auriculated_, having processes
flattened and expanded on either side of the umbones, as in Pecten (cut,
fig. 82). When there is one of these on each side of the umbones, it is
_bi-auriculated_; when only on one side, it is _uni-auriculated_. When the
expansion is very broad, as in Unio alatus (fig. 142 in the plates), and in
the Hammer Oyster (cut, fig. 83), the term _alated_ is used.
[Illustration: Fig. 82, _auriculated_; 83, _auriculated_, _alated_.]
With regard to these alated species of _Uniones_, it is necessary to
observe that they are also "_adnate_," as it is termed; the two valves
being joined to each other by the dorsal edge of the expanded parts, and
united so completely in substance with each other, that they cannot be
separated without being broken. Many other terms are used to express
difference in Bivalves, but being generally applicable to Univalves and
Multivalves, as well as to them, they will be found explained at large in
the alphabetical part of the work.
MULTIVALVE SHELLS.
These are of three different kinds; first, the "_dorsal_," as they are
termed by Linnæus, because they form a ridge in the back of the animal.
They are composed of eight pieces, or separate valves, placed in a
longitudinal series, being joined to each other by inserted lamina, and
named _Articulata_ by De Blainville, on that account. The genus Chiton is
the only example of this kind of Multivalves.
[Illustration: Fig. 84, 85, Chiton. _a_, anterior; _p_, posterior; _d_,
dorsal ridge; _l l_, lateral areas of the valves; _c c_, central areas; _i
i_, inserted lamina; _m_, margin.]
The second kind, M. De Blainville terms the _lateral_ bivalves, the pieces
being placed in pairs on each side of the animal; these compose the
"Pedunculated Cirripedes."
[Illustration: Fig. 86, Anatina.]
They differ considerably in the number and arrangement of the valves; the
small ones, which are found near the peduncle in some species, are
sometimes termed accessary valves; those which form the edge through which
the bunch of Cilia protrude, are termed _ventral_, and those on the
opposite side _dorsal_. The extremity joining the peduncle is the basal, or
anterior; and the upper extremity is the apsiral, or posterior. The
peduncle is the medium of attachment to submarine substances, to which this
well known tribe of shells adhere.
The third kind are termed _coronular_ by De Blainville, and compose the
order Sessile Cirripedes of Lamarck; they consist of a number of valves
placed against each other side by side in a circle, supported on a plate,
or tube, or cup, and closed by an operculum composed of two or more valves.
The _basal support_ is sometimes thick and flat, sometimes forming an
elongated tube, and sometimes hollowed out into a cup. In other species it
is altogether wanting. The operculum always consists of more than one
piece, generally of two pairs: they are either articulated to each other by
serrated edges, and placed against each other conically, as in Balanus, or
they lie flat in two pairs against each other. Through the ventral pair the
_cirrhi_ protrude.
The _parietal_ valves, composing the principal part of the shell, vary in
number, form and position. The _anterior_ valves are placed on the same
side with the cirrhi; the _posterior_, those on the opposite side; and
those which remain between on each side are the lateral valves. In many
cases, particularly in Balanus, each valve is separated into the
_prominent_ and _depressed_ areas, and the inserted lamina. In some
instances, the parietal portion is formed by a single rounded piece.
[Illustration]
In the accompanying cut (87), the prominent areas are distinguished by the
letters _pr_, and the depressed areas by _r_; the posterior valves of the
operculum are marked _p. o._, and the anterior _a. o._ The basal valve
(fig. 88) belongs to a Balanus. Fig. 89 is an Acasta, the cup-shaped base
of which is represented at fig. 90.
In the foregoing explanations we have omitted many of those general terms
which, relating to external characters, are applicable to shells in almost
every division of the system. It may be as well, however, to enumerate a
few of them in this place, although they are explained under their
respective letters in the alphabetical part of the work.
When bars or ribs, or large striæ are crossed by others radiating from the
umbones, shells are said to be _cancellated_, as represented in cut, fig.
When there is a series of nodules or spines on the upper part of thewhorls, they are _coronated_, as shewn in cut, fig. 92. When a series of
projecting parts overlay each other, in the manner of tiles, as in the cut,
fig. 93, the word _imbricated_ is applied. When marked by a regular series
of ridges, radiating from the apex, they are _pectinated_; the species of
Chiton, a single valve of which is represented in cut, fig. 94, has
received the specific name of _pectinatus_, in consequence of this
character. Shells are said to be _plicated_ when characterized by angular
bendings or foldings in their surface, as shewn in cut, fig. 95. A strong
instance of this is seen in the Ostræa Crista-Galli. When the margin of any
shell has a series of minute notches, resembling the teeth of a saw, it is
said to be _serrated_; when covered with raised points or spines it is
_aculeated_; and when striated in both directions, it is _decussated_; when
covered with a number of raised rounded points, it is _granulated_; and
having a series of these points placed in a row, near or upon the edge, it
is _denticulated_, as already explained in reference to the outer lips of
Spiral Univalves. When the external surface is rendered uneven by raised
knobs, it is said to be _tuberculated_; and if rendered rough and prickly
by sharp points it is _muricated_, as in the cut, fig. 97. The term
_reticulated_ is applied to fine raised lines, crossing each other, and
resembling fine net-work.
[Illustration: External surface. Fig. 91, _cancellated_; 92, _coronated_;
93, _imbricated_; 94, _pectinated_; 95, _plicated_; 96, _decussated_; 97,
_muricated_; 98, _foliated_.]
By the foregoing general observations and explanations, it is trusted that
the reader will be prepared for the following exposition of the general
arrangement of Lamarck, and the principles upon which it is founded.
* * * * *
GENERAL ARRANGEMENT
ACCORDING TO LAMARCK.
In Lamarck's "Histoire Naturelle des Animaux sans Vertebres," he divides
the invertebrata into classes, the 9th, 10th, and 11th of which include
animals possessed of shells properly so called. These are the ANNELIDES,
the CIRRIPEDES, the CONCHIFERA, and the MOLLUSCA.
The class ANNELIDES constitutes the 9th, and is divided into three orders,
namely, the "Apodes," "Antennees," and "Sedentaires"; the last of which,
_Sedentaria_, alone contains testaceous animals. This order includes
tubular shells, which, with the exception of Dentalium, are irregularly
twisted, and attached to each other, or to extraneous substances. The first
family _Dorsalia_, contains the genus Siliquaria (plates, fig. 1), known
from the Serpulæ, by the slit which passes through the whole length of the
shell on the upper surface of the tube. The second family, _Maldania_, has
the genus Dentalium (plates, fig. 2), a species of which are commonly known
by the name of "tooth shells"; these are regularly formed, curved conical
tubes, open at both extremities. The third family, _Serpulacea_, includes
the genera Serpula, Spirorbis, Galeolaria, Vermilia, Spiroglyphus, and
Magilus. The only shell that a learner would be likely to place among these
incorrectly, according to the system, is the Vermetus (plates, fig. 345),
which being regularly spiral at the apicial extremity, has been placed
among the Mollusca; to which situation the whole of the shells under
consideration have a better title than is generally supposed. It should be
mentioned that the Serpulacea are provided with opercula.
Class CIRRIPEDES.pg.1
Class CIRRIPEDES.
This class constitutes the tenth of invertebrated animals, and receives its
name from the jointed and ciliated branchia which protrude between the
opercular valves. They are Multivalve shells, and were all included in the
single genus Lepas in the system of Linnæus, and are commonly known by the
name "Barnacles." Lamarck has, however, divided them into two distinct
orders. First, the _Sessile_ Cirripedes or those which being composed of
several valves, joined to each other, side by side in a circle, are
attached to each other, or to submarine bodies by the basal portion of
their own substance, and form a hollow, irregular cone, with the aperture
above closed by an operculum consisting of two or more valves. Secondly,
the _Pedunculated Cirripedes_, which are composed of valves placed in pairs
against each other, so as to form a flattened disc attached by means of a
tendinous tube called a peduncle. The first of these orders includes the
genera Tubicinella, Coronula, Platylepas, Clitia, Conia, Elmineus,
Catophragmus, Octomeris, Balanus, Creusia, Nobia, Savignium, Pyrgoma, Adna,
Megatréma. The second contains the genera Pentelasmis, Scalpellum, Smilium,
Pollicipes, Bisnæus, Lithotrya, Ibla, Octolasmis, Cineras, Otion.
Conchological writers are not agreed as to the propriety of allowing the
above to enter into the present science.
Class CONCHIFERA.
The shell of a conchiferous animal is always bivalve, composed of two
pieces placed opposite to each other, joined at the dorsal margins by an
elastic hinge. All true bivalve shells belong to animals of this class; and
the correspondence between the shell and the animal is so true that on
examining an empty bivalve shell we can not only determine that its
inhabitant belonged to this class, but also decide on the particular order
and family in which it should be placed, without seeing the soft parts.
The first general division of Conchifera is that which results from
observing the muscular impressions, or marks made on the inner surface of
the valve by the insertion of the adductor muscles. All Conchifera are
divided into two orders, as follows:
First Order, _Conchifera Dimyaria_.
Having two adductor muscles, and consequently two impressions in each
valve. They are separated into the following families:
_Tubicolæ_ (plates, fig. 44 to 54), having shelly tubes besides the
valves. This family contains the genera Aspergillum, Clavagella,
Teredina, Teredo, Xylophaga, Fistulana, and Gastrochæna.
_Pholadaria_ (plates, fig. 55 to 59), cylindrical, living in holes
in rocks pierced by the animals. Lamarck places in this family the
genera Pholas and Gastrochæna, the last of which belongs more properly
to the family Tubicolæ, as placed above.
_Solenacea_ (plates, fig. 60 to 68), longitudinally (transversely,
Lam.) elongated, open at the anterior and posterior extremities. This
family contains the genera Solen, Pholadomya, Panopæa, Glycimeris
(Solecurtus) and Solenimya.
_Myaria_ (plates, fig. 69 to 76), ligament internal. A spoon-shaped
ligamentary pit in one or
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