A Conchological Manual - George Brettingham Sowerby (any book recommendations .txt) 📗
- Author: George Brettingham Sowerby
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variations, too minute and particular to be described in this part of the
work.
COLUMELLAR LIPS.
[Illustration: Fig. 60, Melo, _obliquely plaited_; 61, Turbinellus,
_horizontally plaited_; 62, Ancillaria, _varicose_, _tortuous_; 63, Natica,
_straight_.]
OPERCULUM.
The aperture of many species of shells remains constantly open; but in a
great number of species it is occasionally closed, whenever the animal is
retracted within the shell, by a calcareous or horny piece called the
operculum. This must be distinguished in the first instance from another
kind of calcareous covering, which in some univalve shells serves to close
the aperture during a certain portion of the year. This piece, named the
_epiphragm_, although hardened and shelly in appearance, is no real part of
the animal or of the shell; being only a secretion temporarily hardened,
for the purpose of defending the animal from external influences during the
_hibernating_ or _torpid_ season, to be dissolved when that season is at an
end. On examining this piece, it will be observed that it is not formed in
regular layers like the rest of the shell; while the true operculum is of a
regularly laminated structure, having a nucleus and receiving obliquely
deposited additions, either in a lateral spiral or concentric direction. It
is attached to the posterior part of the foot on the upper surface; and
when the animal retires within its shell, that part of the foot enters
last, drawing the operculum after it, and thus closing the aperture.
The opercula of various shells differ in the first place as to their
chemical composition. They are _calcareous_ when formed principally of
calcareous matter, like the rest of the shell, as in Neritina, Nerita, and
some others. They are _corneo-calcareous_, when upon an internal lamina of
horny consistency there is a thickened layer of shelly matter. This is the
case with shells of the genus Turbo and Phasianella, which are on this
account distinguished from those of the genus Trochus; the opercula of the
latter being horny or _corneus_.
The size of the operculum is distinguished by comparison with the rest of
the shell; thus, those of Strombus, Cassis, &c. are small; while those of
Cyclostoma and others are large, filling up the cavity at its outer edge.
The direction in which the successive layers are deposited, forms another
ground of distinction. The disc is formed in some instances of a series of
whorls, the apex or nucleus being more or less central; if these whorls are
numerous, the operculum is described as _multispiral_, as in shells of the
genus Trochus; if few, as in Cyclostoma, it is _paucispiral_. In some
instances the flattened spire consists of but one whorl, it is then
_unispiral_; and when scarcely one turn is completed, it is described as
_subspiral_. When the layers are applied upon each other in such a manner
that the nucleus is central, and the edges of the subsequent layers are
extended beyond each other all around, so as to form rims, the operculum is
described as _concentric_; if the nucleus is lateral, or at one side
without being spiral, it is _lammellated_; and when it forms a terminal
point, enlarging in the form of a finger-nail or claw, it is
_unguiculated_. In the operculum of a Neritina, there is a lateral process,
by means of which it is locked into the columella, the term _articulated_
is then applied. In that of Navicella, there is also a process which
appears to radiate from the nucleus, it has therefore been described as a
_radiated_ operculum.
_Opercula of Spiral Univalves._
[Illustration: Fig. 64, _multispiral_; 65, _paucispiral_; 66, _concentric_;
67, _articulated_; 68, _radiated_; 69, _lammellated_; 70, _unguiculated_.]
BIVALVE SHELLS.
Bivalve shells, named Conchacea by Lamarck, are those which consist of two
principal portions united to and folded upon each other by means of a
hinge. The pieces united compose the shell, while each piece separately is
called a valve. Considering the bivalve shell as a whole, it will be
necessary, in the first instance, to describe the position in which it is
to be observed, in order to give the student a clearly defined notion of
what is intended, when terms expressive of height, depth, length, breadth,
&c. are used, as well as when the anterior and posterior extremities are
spoken of. For this purpose, we must suppose the animal to be living and
creeping along the bed of the sea by means of its foot; where this foot
protrudes, will be the _ventral margin_, and the opposite part the _dorsal
margin_ of the shell. There will then be a valve on each side; and if we
further suppose the animal to be walking forward with its back to the
observer, the _right_ and _left_ valves will correspond with his right and
left sides.
_Measurements._
[Illustration]
The _length_ will be measured from _anterior_ (_a_) to _posterior_ (_p_),
and the lines of growth running in the same direction will consequently be
_longitudinal_ or _concentric_; _transverse_ of some authors. The height
will be from the umbones (_u_), to the _ventral margin_, and lines or bands
in that direction are termed _radiating_; longitudinal, according to some
authors.
The points from which the growth of the shell commences, are called the
_umbones_; these usually turn towards the anterior part of the shell: if
this circumstance fails to point out the anterior, it may in many cases be
distinguished by the muscular impressions of the mantle. If this has a
sinus or winding, it is always near the posterior muscular impression; and
in all cases where there is an external ligament, it is on the posterior
side.
There is sometimes an impression near the front of the umbones, which forms
a semicircle on each valve; the space within this semicircle is called the
_lunule_ (wood-cut, fig. 71 and 72, _l. l. l._); a corresponding
depression, when it exists on the posterior margin near the umbones, is
named the escutcheon.
_Hinge._
[Illustration: Fig. 73, _l t_, lateral teeth; _c t_, cardinal teeth; _c_,
cartilage under the ligament; _l_, ligament; _f_, fulcrum of the ligament.]
The _hinge_ of the shell is on the _dorsal_ margin, and is composed of the
various apparatus by which the two valves act upon each other in opening
and shutting. It consists of a _ligament_, which is placed on the dorsal
margin, just at the back of the umbones, and unites the two valves
together; the _cartilage_ or thick gristly elastic substance, sometimes
found close to the _ligament_, to which it then forms an inner coating, and
sometimes received into a pit within the shell. It serves the purpose of
keeping the shell open when not forcibly closed by the adductor muscles. An
inner layer of shelly matter upon which are placed teeth, and pits to
receive them on the two valves reciprocally. Each of these it will be
necessary to treat of more at large; observing, at the same time, that in
some species of Bivalves these parts may be wholly or partially wanting.
Thus we meet with some shells, such as the Muscle, without teeth; and there
is the group containing Pholas, &c. the hinge of which is destitute of
teeth and ligament, the two valves being kept together by loose cartilages,
and by the contracted space in which they are confined.
_Ligament and Ligamentary Cartilage of the Hinge._
These two distinct substances have been described by many writers as
though, composing the same mass, they were of one substance; but the
difference may very easily be explained. The _true ligament_ is external,
being fixed on the edge of one valve behind the umbones, and passing over
in an arch to the corresponding edge of the other, very correctly retaining
the name of _ligament_, because it serves the purpose of binding the two
together. The thick, elastic substance, which Mr. Gray names the
_cartilage_, is sometimes found in connexion with the ligament, so as to
form one mass with it, although it is always separable and placed within
it: it is sometimes placed quite within the shell, and separated from the
ligament, in a pit or hollow formed for its reception in the hinge lamina,
near the centre. It is found in both valves, and being elastic, the portion
in one valve presses against that in the other, so as to keep the valves
apart, unless voluntarily closed by the adductor muscles of the animal. The
ligament is sometimes spread over an external area, as in Arca, while the
cartilage is placed in several grooves of the same area, beneath the outer
covering.
_Hinge lamina, Teeth and Fulcrum of the Ligament._
In a great variety of cases, there is a thickening of the substance of the
shell within, under the dorsal margin; this is named the hinge lamina. It
is sometimes merely callous; but in many cases it has raised _teeth_ in
both valves, those in one valve entering into corresponding cavities in the
other. Those which are placed immediately below the umbones, and seem to
take their rise from beneath them, are called _cardinal teeth_; those at a
distance from the umbones, which are seen to lie along the upper margin of
the shell are named _lateral teeth_.
When the cardinal teeth terminate in a double point, which is not
unfrequently the case, they are said to be _bifid_. The lateral teeth, in
various species, are distinguished as terminating _near_ to, or at a
_distance_ from the umbones. In the Nuculæ and Arcæ there is a row of teeth
placed across the hinge lamina. In which case, the lateral cannot be
distinguished from the cardinal teeth.
_Muscular Impression._
[Illustration: Fig. 74. _a, anterior; p, posterior; m i, muscular
impressions._]
Lamarck divides the Bivalve shells into two general orders; the first is
named "Dimyaria," having two adductor muscles; and the second,
"Monomyaria," having but one. These adductor muscles are used for the
purpose of drawing the valves together, being composed of contractile
fibrous gristle, fastened firmly to the inner surface of each valve. The
place where they are thus fixed may be seen when the animal is removed, by
depressed areas, which are generally pretty well defined, and are named
_muscular impressions_. Where there is but one adductor muscle, there will
be but one of these impressions near the centre of each valve, but in the
Dimyaria, where there are two, the impressions are seen, one on the
anterior, and one on the posterior of each valve, just below the _hinge_
lamina. They are sometimes _complex_, that is composed of several portions
in a group; but in general, they are simple and well defined.
They are also described as large or small, in proportion to the size of the
shell; regular or irregular in form. The animal is attached to the inner
surface by the fibrous portions of the mantle, which creates a linear
impression or _cicatrix_, commonly described as the _palleal impression_,
or muscular impression of the mantle. It runs near the ventral margin from
one muscular impression to the other, sometimes in a smooth _continuous_
line or band, and sometimes in an interrupted series of small impressions.
Near the point of union with the posterior muscular impression, there is
sometimes a more or less considerable winding inwards towards the centre of
the shell, and back again towards the point of union. This is named the
_sinus_, and is distinguished as being _angular_ or _rounded_, large or
small, according to the species. When it enters towards the centre of the
shell in a tongue-shaped outline it is said to be _ligulate_. Where it
exists it affords a certain index to the posterior side of the shell; as it
is the region through which the excretory tubes pass.
_Umbones._
These are the prominent points of the dorsal edge, where the growth of the
shell commenced, and are called beaks, by some English writers. In some
instances they are close to each other; in others they are rendered distant
from each other by the intervention of areas in the hinge, as in Spondyli,
&c. In Pectunculus they are _straight_; in Venus _curved_ towards the
anterior margin; in Isocardia, _spiral_; in Chama, _decumbent_; in Diceras,
_free_. In shells subject to external corrosion, the process commences at
the umbones.
[Illustration: Fig. 75, _distant_; 76, _straight_; 77, _curved_; 78,
_spiral_; 79, _decumbent_; 80, _free_; 81, _close_.]
GENERAL CHARACTER OF BIVALVES.
When the _breadth_ is spoken of, the distance between the most convex parts
of both valves, when closed, is intended; but when an expression implying
_thickness_ is used, it refers to the substance of each valve: it is
important to bear this in mind, as many persons have been misled by
descriptions in which the distinction has not been attended to. Glycimeris
(fig. 67 in the plates) is a _thick_ shell, but Anatina (fig. 69 in the
plates) is a _broad_ one.
_Regularity._
A great number of Bivalves are extremely regular in their form. These are
generally locomotive, and consequently free from those obstructions in
growth occurring to stationary shells, which being confined in a particular
position, or to a particular spot, modify their shape according to the
substance with which they come in contact, and thus become irregular. This
is
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