A Conchological Manual, George Brettingham Sowerby [little readers .TXT] 📗

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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