Part 2

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.

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 regular spire only taking place after the few first volutions.

SPIRAL NON-SYMMETRICAL UNIVALVES.

As these constitute the largest class, it will be necessary to dwell upon them in detail. First as to _measurement_.

The length is measured from the apex, to that part of the aperture _a_ (fig. 24), at the greatest distance from it. The _breadth_ is in the opposite direction. The _anterior_, or front part of the aperture, is marked _a_, where the head of the animal protrudes.

_Spire of non-symmetrical Univalves._

In counting the whorls of which the spire consists, we commence at the apex, and reckon downwards to the last, or body whorl. The spire is described as being long or short in relation to the aperture: in which case, all that is above the aperture is measured with the spire. Its apex requires particular notice, as the character of the whole shell frequently depends upon the particulars observable in this part. It is sometimes _obtuse_, or blunt; sometimes _acute_, or sharp. In the Cones it is frequently flat, and in Planorbis it is concave. It is sometimes of a different structure from the rest of the shell, retaining the horny and transparent appearance which characterized it when the animal was first hatched. The Tritons present an instance of this, although it is not always observable, owing to the tenderness of the substances which causes it to break or fall away in many specimens. A very remarkable instance also occurs in Bulinus decollatus (cut, fig. 27, 28), so named, because the apex, to the depth of several whorls, falls off, and the shell is _decollated_. In this, and many more instances, among Pupæform land shells, the occurrence of this circumstance seems to be by no means rare or accidental, a provision having been made for filling up the opening by a septum. A _papillary apex_ is one which is swelled at the extremity into a little rounded nob, or nipple; and a _mammellated_ apex is one which is rounded out more fully into the shape of a teat.

_Whorls._

The spire is described as consisting of _numerous_ or _few_ whorls, and sometimes the number of them is particularly stated. A whorl consists of one turn of the spiral cone. The whorls are described as _flattened_, when the sides are not bulged out so as to cause the outline of the spire to deviate considerably from straightness: when the contrary is the case, the whorls are said to be _ventricose_, and either _rounded_ or _angulated_. The degree of rapidity with which the whorls become enlarged presents an important source of distinction. The _suture_, or seam, which separates one whorl from another is also noticed as being _distinct_ or otherwise; _canaliculated_, or grooved; or covered by an enamel, which in some instances is swelled into a ridge or _tumid_.

_Suture._

_Varices._

Varices are caused by periodical rests or stoppages in the growth of the shell, when the edge of the aperture thickens, and renders the shell as complete as when full grown. Again, after an interval, another check takes place, and another thickened edge is formed, and so on in succession, until the animal arrives at maturity, and the shell is full-grown. The thickened edges successively forming the aperture, remain visible on the outside, through all the subsequent stages. When these rests take place at frequent periods, the varices will of course be numerous as in Harpa and Scalaria. They occur at regular or irregular distances, varying in shape and other characters. When the varices occur at regular intervals, and form a connected ridge from whorl to whorl up the spire, they are said to be _continuous_, as in Ranella; when on the contrary, the varix on one whorl does not come in contact with that on the other, they are described as _discontinuous_. In order to distinguish a regular varix from a mere external ridge, it will be sufficient to notice whether its edge overlaps the external surface, and whether it resembles the open edge of the aperture, which true varices do.

_Aperture._

The aperture or opening of the spiral tube, was formerly described as the mouth; a term calculated to convey an erroneous impression, when applied to a part of the shell which has no correspondence with the mouth of the animal. The word _aperture_ is used by modern writers in a general sense, including the cavity, its edge, and the canals. The cavity itself is distinguished in various shells as to its shape, which depends much upon the degree of modification produced by the last whorl. In some cases, as in Cyclostoma, where the aperture stands apart from the last whorl, the shape is round, or nearly so. The Scalaria presents a good example of this. In others, where the inner edge or lip, wrapping over the body whorl is nearly straight, the aperture is _semi-lunar_, or half-moon shaped: this is remarkable in the "_Neritacea_" of Lamarck, named, on that account, "_hemi-cyclostomata_" by De Blainville. In a great number of instances, the lower part of the body whorl enters obliquely into the upper part of the aperture, the result being a _pyriform_, or pear-shaped opening. The aperture is described as _long_ when it is largest in the direction of the axis, and _wide_, in the contrary case. The _anterior_ is the part at the greatest distance from the apex, and the body whorl; the _posterior_, the part nearest to the apex. Thus some apertures are described as _posteriorly contracted_ and _anteriorly widened_, or the reverse. A _linear_ aperture is one contracted in its whole length, as in Cypræa. When the whorls are angulated, a _trigonal_ aperture is the result, as in many species of Trochus. Some are _transversely oval_, that is in an opposite direction to the axis, and others _longitudinally oval_. When the whorls are formed with two outer angles, a somewhat quadrated aperture is formed. There are other variations too numerous to mention.

_Apertures._

The entire edge of the aperture described generally, is named the Peritrême, but this term can only be conveniently applied in cases where, in some at least of its characters, it is the same all round, so that one descriptive term is applicable to the whole. As, however, this is of rare occurrence, it is found convenient in descriptions to separate the rim from the outer lip. In a great number of instances, this is done naturally, by a canal, or notch at the anterior or lower extremity, and by the posterior union of that part which overlays the body whorl with the other portion. At these two points the outer and inner lips separate from each other: we therefore describe the

_Canals of the Aperture._