A Conchological Manual - George Brettingham Sowerby (any book recommendations .txt) 📗
- Author: George Brettingham Sowerby
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corrosion near the umbones. There are but few genera of fresh-water shells
besides the Uniones, among bivalves, and the "Melaniana" among univalves.
Concerning the former it may be observed, that they are all pearly within,
and the colour of the thick horny coating embraces all the varieties of
brownish and yellowish green.
The MARINE, or _sea-shells_, belong to all the classes and orders, and
include by far the greater number of species. They vary in the habits of
the animal, and consequently in the situations in which they are found.
Some are found buried in sand and marine mud, and are named "_Arenicolæ_"
or inhabitants of sand; others in holes of rocks and other hard substances,
then they are named "_Petricolæ_,"--some of these latter form the holes in
which they live by corroding or eating away the stone. A section of these
form the family of "_Lithophagidæ_," or stone-eaters, of Lamarck. Others,
again, take up their parasitical abode in the bodies of animals, and feed
upon their substance; as for instance, the Stylifer, which is found in the
vital part of star-fish, and Coronula, and Tubicinella, found buried in the
skin of the whale.
LOCOMOTION--_Attached, Unattached._
A much more subordinate source of distinction arises from the freedom or
attachment of the shells. Some of them float or walk freely in their
natural element; others are fixed or attached to foreign bodies. Among
those which are attached, there is again a difference as to the mode of
attachment. Some are united to foreign bodies by means of a glutinating
substance, secreted by the animal, and joining part of the surface of the
shell to that of the stone, coral, or other substance. In this way shells
are fixed to each other in groups; this is the case with the Spondyli among
bivalves, and the Serpulæ among univalves. M. de Blainville applies the
term "_Fixæ_" to these shells. Others are kept in a particular place by
means of a _Byssus_ or Tendinous fibrous line or bunch of silky hairs,
acting as a cable, and allowing the Mollusc to ride as it were at anchor.
This Tendon is connected with some part of the animal from which it passes
through an opening or hiatus in the shell, as in the Terebratula and the
Mytilus.
In the former, represented by the cut, fig. 8, the tendon passes through a
perforation in the upper valve; and in the latter, Mytilus, fig. 9, the
byssus passes out between the valves.
Before proceeding to explain the characters of the different groups,
according to the modern system of classification, it may be desirable to
explain the terms by which the different parts and characters are
described, and to shew the manner in which the shells are measured. For
this purpose we shall treat of the general divisions separately. We begin
with
UNIVALVE SHELLS.
In considering Univalves merely with reference to their mathematical
construction, the first point demanding our attention is, whether they are
symmetrical or non-symmetrical, or, in other words, whether a straight line
drawn through the shell would divide it into two equal parts. The greater
part of univalves are non-symmetrical, being rolled obliquely on the axis;
but many are symmetrical, being rolled horizontally on the axis. The
Nautilus presents an illustration of the latter; the Snail is a familiar
example of the former.
_Symmetrical Univalves._
In describing these it will be well to commence with the most simple form,
such as the Patella,--taking a conical species as an example. In this it
will be observed that there is no winding or curvature, but a simple
depressed cone, and that the line _a_, _p_, divides it into two equal
parts.
The _anterior_, _a_, (_cut_, fig. 10) is known by the interruption of the
muscular impression which surrounds the central disc (_d._) This
interruption of the muscular impression is in the place where the head of
the animal lies in the shell. The impression itself is caused by the
fibrous muscle which attaches the animal to the shell. The apex (_a_) in
Patella, generally leans towards the anterior (_a_) part of the shell, and
away from the posterior (_p_); and this circumstance has caused some
mistakes, because in Emarginula the apex leans towards the posterior; and
students, instead of examining the muscular impression, which is the only
criterion, have only noticed the direction in which the apex turned, and
concluded that to be the anterior, towards which it inclined. The lines or
ribs running from the base to the apex of the shell, in the direction _r_,
are called _radiating_ lines; and those which encircle the cone in the
direction _c c_, from front to back, are very properly described as
_concentric_. The _length_ is measured from front to back in the line _e_;
the breadth, from side to side, in the line _b_; and the depth from the
apex to the base.
Let it be observed that patelliform, or limpet-shaped shells are not all
symmetrical; Umbrella, Siphonaria, Ancylus, &c. will form exceptions, of
which we have yet to speak. And the learner may also be reminded that the
Limpets themselves are not _all_ regular in their form: for as they adhere
to rocks and other rough surfaces, and are so little locomotive, in many
instances they partake of the inequalities of the surface, and conform to
its irregularities. This adherence is not effected by any agglutinating
power in the animal, nor by any tendinous process like that described
above; but simply by means of the foot of the animal acting as a sucker.
The next variation in symmetrical univalves is to be observed in the
tubular, curved form, the example of which will be the Dentalium, fig. 12.
[Illustration: Dentalium Elephantinum.]
This has an opening at the anterior termination _a_, called the aperture.
The opening at the posterior end (_p_) is named a fissure, or perforation.
The ribs running along the sides of the shell are _longitudinal_, or
radiating. And the lines round the circumference are _lines of growth_, or
_concentric_--each one having in succession, at earlier stages of growth,
formed the aperture. They are described as concentric, or transverse.
_Symmetrical Convolute Univalves._
The Nautilus, the Spirula, the Scaphite, and the Ammonite are the leading
types of this form; but when we use the term symmetrical, in reference to
these, the word must not be understood in its strictest sense, for no shell
is _perfectly_ symmetrical: but it means that there is no perceptible
difference in the proportion of the two sides; as in the human body, the
right side is larger and more powerful than the left, yet to a degree so
small that it gives no apparent bias to the figure.
CHAMBERED SHELLS.
Many of the shells now under consideration are chambered, that is, the
internal cavity is divided into separate compartments by plates reaching
across it, named _Septa_; and the only connection between the chambers is
formed by the small pipes passing through them, to which the name of Siphon
is attached.
_Septa._
The septa are _simple_ in some species, as in the Nautilus, fig. 13. In
others they are _undulated_, having waved edges, as in some species of
Ammonites; in others they are _angulated_, as in Goniatites, fig. 480 in
the plates; and in the greater number of instances, among the Ammonites,
they are _arborescent_, or branched.
In the above section of a Nautilus, fig. 13, diminished in size, showing
the whorls and chambers (_c_), it will be seen that the edges of the septa
(_s_) are formed in one simple curve. In fig. 14, the upper part of an
Ammonite, the undulating line will be seen; and in fig. 15 a specimen is
given of the arborescent septa.
_Siphon._
The Siphon is _dorsal_ when placed near the outside of the whorls;
_central_ when near the middle; and _ventral_ when near the inside of the
whorl, or that part which leans against the last volution. When it passes
uninterruptedly from one chamber to another, it is described as
_continuous_, as in the case of Spirula; when, on the other hand, it only
passes through the septum a little distance, and opens into the chamber, as
in Nautilus, it is _discontinuous_.
_Whorls of Symmetrical Univalves._
They are _disunited_ when they do not touch each other, as in the case of
Spirula (fig. 471 in the plates); but in the contrary case they are said to
be _contiguous_. In some species of Nautilus the whorls overwrap each other
in such a manner that the early whorls are entirely covered by the last,
the edges of which reach to the centre of the disk: the spire is then said
to be _hidden_; as in the Nautilus Pompilius. In Nautilus umbilicatus the
spire is nearly hidden, the whorls not quite covering each other; but in
the greater number of the Ammonites, the largest part of the preceding
whorls is seen. To express the degree in which the whorls overwrap each
other, has caused much difficulty in concise descriptions. Perhaps it would
be well to apply the term _spiral disc_ to so much of the shell as is seen
besides the last whorl, and to describe it as large or small in diameter,
compared with the whole: or to say that the whorls of the spire are half,
or one-third, or one-fourth covered, as the case may be.
_Aperture of Symmetrical Univalves._
In Ammonites Blagdeni and some others the aperture is of an oblong square;
it is then said to be _sub-quadrated_; in Nautilus triangularis it is
_angulated_; in Ammonites Greenoughi it is of an interrupted oval shape,
described as _elliptical_. In the greater number of Orthocerata, it is
rounded or _circular_. The entrance of the last whorl into the aperture of
some rounded species of Nautilus causes it to take a _semi-lunar_ form; if
rounded at the sides it is said to be reniform or kidney-shaped; if pointed
at the sides it is _semi-lunar_; and in some species of Ammonites, it is
five-sided or _quinque-lateral_.
_Measurement of Symmetrical Conical Univalves._
The _width_ is measured across the aperture, which is the widest part of
the shell. The _length_ (_l_) from the dorsal part (_d_) of the aperture to
the dorsal part of the _whorl_ (_d_) on the opposite part of the shell. The
_ventral_ part of the whorls is that nearest to the axis, and the _dorsal_
that which forms the outline of the figure.
NON-SYMMETRICAL UNIVALVES.
These are _conical_, _irregular_, _spiral_, or _convolute_. The _conical_
form is when there is no enrolment of the apex. Although the Patellæ were
described as symmetrical, there are several species of Patelliform shells
which are not symmetrical. In Umbrella, for instance, the apex is oblique,
the shells being placed obliquely on the animal. In the genus _Siphonaria_,
there is a groove on one side, where the brachia or gills of the animal
rest. In the genus Ancylus, it will be observed that the apex bends on one
side, and the animal is like the Limnæa, which has a spiral shell. The cup
and saucer Limpets, or Calyptrædæ, present a group which requires to be
described, differently from the symmetrical or true Limpets. Their
structure is very curious, and they vary considerably among themselves,
some of them being simply conical, others nearly flat, or discoidal, and
others more or less spiral. But their principal peculiarity consists in
their having a small internal process or plate variously shaped, commonly
named their _septum_.
_Septa of Limpets._
The septa of Limpets assume a variety of forms, the principal of which will
be seen in the accompanying engravings.
The form from which the group derives its generic appellation is that of
the cup-shaped or _Cyathiform_ species (fig. 17). In the Crepidulæ, or
Slipper-Limpets, the septum is flat, reaching across the opening, like the
deck of a vessel; it is then described as _transverse_ (fig. 20). In
Calyptræa Equestris, it has two prominent points, and is described as
_bi-furcated_ (fig. 18). In another species, it is a three-sided plate
rather spiral at the apex (fig. 19).
_Measurement of Cup and Saucer Limpets._
The line marked _a_, _p_, _ll_ indicates the direction in which the shell
is to be measured for _length_. _a_ indicates the _anterior_, _p_ the
_posterior_. The line _d_ (fig. 23), from the apex to the base, is the
_depth_. The line _b_ (fig. 28), is in the direction of the breadth.
_Irregular non-symmetrical Univalves._
Serpuliform shells are irregularly twisted (_tortuous_) hollow tubes, which
were formerly considered to have been secreted by a kind of worm, but now
known to be the shells of true Molluscs, of a kind not very widely
differing from those which have regularly spiral shells. The greater part
of these are attached to foreign bodies, or to each other in groups. Some
are attached by the whole length of the shell, they are then said to be
_decumbent_. Some of these are coiled round like the Spirorbis, the little
white shell seen on the carapace of the Lobster or on leaves of sea-weeds;
they are then said to be discoidal; others again, such as the _Vermetus_,
approach more nearly to the spiral form. The deviation from the
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