Part 4

The classification of shells, that is, their systematic arrangement into classes, orders, families, genera and species, cannot be made to depend entirely upon the characters observable in them, viewed by themselves; for this reason, that many similarly formed shells form the habitations of animals perfectly distinct, and that many molluscous animals are found to agree with each other in every respect but in the form of their testaceous support. There are, however, many important distinctions to be observed in the shells themselves, leading to the establishment of many of those very divisions, which would afterwards be confirmed by an examination of the soft parts. It is necessary to attend, as far as means and opportunity will allow, to all the points of difference, both in the shell and in the animal, in order to form, and in some instances even to appreciate, a generic or larger distinction. It will therefore be our endeavour to explain the general principles upon which those distinctions are formed, and the manner in which they are applied and expressed in detail by scientific writers.

NUMBER OF PIECES, OR INDEPENDENTLY FORMED PARTS.

The first, most simple and obvious division of shells, is that which results from the number of separate pieces composing them. Hence the distinction implied by the terms UNIVALVE, or consisting of a single piece; BIVALVE, or composed of two pieces; and MULTIVALVE, or composed of more than two. For an example of univalve, take a common whelk; for a bivalve, take a muscle or a scallop; and for a multivalve, the barnacle, or balanus, found adhering to the common oyster.

But although this arrangement may appear at first sight perfectly easy and plain, some explanation will be necessary in order to guard the student against understanding the above expressions in their strictest sense, without qualification. Thus the univalves are said to consist of a single piece, or spiral cone; but it would be more correct to speak of this piece as forming either the whole or the principal part of the shell: for there is in many instances, a much smaller flattened piece attached to the foot of the animal, which being drawn in when it retires, closes the aperture as with a kind of door, to which in fact the word valve might be very properly applied; it is called however the OPERCULUM, and the little horny plate, frequently drawn out by means of a pin from the aperture of a periwinkle, will present a familiar example.

The same may be said respecting the bivalves; for besides the principal portions or valves of which the shell is composed, there are in many species, one or two smaller separate portions, named "accessary plates" by some authors. They are fixed by means of cartilages, on the back of the hinge.--The engraving, fig. 7, represents the accessary valves of a species of Pholas, which was on this account arranged by Linnæus with the Multivalves. Nearly allied to the Pholades is a set of shells to which De Blainville has given the name "Tubicolæ," or inhabitants of tubes. In this case, the bivalve shell is connected with a testaceous tube or pipe, to which it is attached either by one or by both valves, or in which it lies attached only by the cartilages of the animal. In the genus Aspergillum, the two small valves are soldered into the sides of the tube in such a manner as to constitute a part of it. One of these shells, called the Water-spout, might be taken up by a person not aware of its real nature, and regarded as a pipe or tube prettily fringed, and nothing more; but upon a closer examination, he would find the two valves, the points of which are visible from the outside of the tube.

HABITS--Land, Fresh-water, or Marine Shells.

Another distinction, leading to important results in classification, is that which is derived from the nature of the element breathed by the Mollusc. And although this consideration belongs more especially to the study of the animal itself, yet the habits of the animal materially influence the structure of the shell.

The TERRESTRIAL or LAND Molluscs live on land, breathe air, and feed on plants and trees.--Those who find pleasure in horticultural pursuits will at once call to mind a too familiar example of these Molluscs in the common garden snail. The Land-shells are all univalves, and constitute a family in the Lamarckian system under the name "Colimacea," or snails, corresponding with the Linnean genus Helix.--They are generally light in structure and simple in form.

The AQUATIC, or Fresh-water Molluscs, such as the Planorbis, commonly called the Fresh-water Snail; the Unio--known by the name of Fresh-water Muscle, is found in ponds, ditches and rivers. The epidermis of these is generally of a thick, close-grained character, and they are subject to 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.

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.