INTRODUCTION.
The Science of Conchology affords a very delightful and instructive amusement for the leisure hours of those who, retiring occasionally from the gaieties of fashionable life, seek pleasure in the quiet contemplation of some of the smaller, but not less wonderful operations of creative wisdom. And, although the study of shells would be more complete, and rank higher in the scale of philosophical pursuits, were it always accompanied by that of the animal inhabiting them, it nevertheless presents means of intellectual gratification, to many who cannot follow it beyond the cabinet and the boudoir. These may examine with admiration and mental improvement, the beautiful colouring and architecture of these wonders of the deep, they may exercise their taste and judgment in the selection and arrangement of specimens, and their discrimination in detecting and appreciating the distinctions upon which the arrangement is founded.
It is but little that can be known of the subject without forming a collection of greater or less extent; for, as it would be uninstructive merely to delight the eye with the bright colours and elegant form of shells, without possessing correct information respecting them, so it would be insipid and useless to learn technicalities without being acquainted by personal observation with the subjects to which they are applied. The first endeavour should, therefore, be to obtain a few shells as examples of the larger divisions, and, when these are understood, to proceed with the smaller groups, until a collection be formed to represent as many generic forms as possible. It may be as well here to advise those who are forming a collection to be very particular in every practicable instance to have the shells properly named at the time of purchasing; as it will save much trouble, and materially assist in the attainment of the desired object. To this end, recourse should be had to those naturalist tradesmen, who unite the attainment and diffusion of real scientific knowledge with their commercial pursuits.
Supposing, however, that the person who desires to learn the science, possesses a small parcel of unarranged and unnamed shells, without any previous acquaintance with the subject, the following introductory explanations, are drawn up with the view of enabling him, without further assistance, to obtain a general insight into its principles, equal to that of those who have studied it long and laboriously. To effect this, he must read them, carefully comparing the descriptions with the figures referred to, and with the specimens which he may have at command.
After describing the nature of the science and defining its objects, we shall proceed to explain the structure of those objects, and the manner of their growth. We shall then enter somewhat minutely into the principles of classification, the distinctions upon which they are founded, and some of the technical terms used to express them. After which we shall pass through the arrangement of Lamarck, defining the general divisions adopted under the terms of "Classes, Orders, and Families," as far as they are capable of definition. The subdivision of the latter into genera will only be entered into so far as to enumerate the principal of them, the more minute descriptions being reserved for the alphabetical part of the work.
Let none be discouraged by the number of generic distinctions proposed and adopted in modern times; for if well defined, they will be found to facilitate rather than encumber the science. The knowledge of species must be the foundation of every system, and the greater their number, the more necessary it becomes to subdivide them; if, for instance, all the species now known were to have been included in the 50 genera of Linnæus, a single genus would have contained many hundreds of incongruous species, in which case it would be much more difficult to remember them, than if they were to be divided into a far greater number of genera. Every well marked division, however arbitrary its limits, tends to simplify the subject, and to facilitate the researches of the student.
NATURE OF THE SCIENCE.
Conchology is the study of shells, viewed and described as to what they are either in themselves, or in relation to the soft, inarticulate animals which produce them, and of which they form a part. These animals are called Mollusca, and perhaps the best general description of them will be found in De Blainville's "Manuel de Malacologie et de Conchyliologie." The following is a translation, "Animal in pairs, the body and its appendages soft, inarticulate (not jointed), enveloped in a muscular skin, commonly called the mantle, which is extremely variable in form, and has developed either within or upon it a calcareous portion, consisting of one or several pieces, commonly called a SHELL."
The term Mollusca was formerly restricted to those soft animals which were destitute of shells, although possessing in other particulars, the characters described above, and it was used in order to distinguish them from the Testacea, which were covered or internally supported by calcareous parts. In the system of Linnæus, the soft portions are first arranged under the general designation of "Vermes Mollusca," and described without regard to the presence, absence, or character of the shells; and then the shells are separately characterized under the appellation of "Vermes Testacea," without any further notice of the animal, than an indication of the genus to which it belongs; thus the animal of Cypræa is said to be a Limax, and that of Tellina a Tethys.
The nearest approach to correctness, and the most philosophical method of study will be found in the modern system, adopted by Lamarck and his followers, of observing these animals as a whole, and arranging them according to the assemblage of characters which they present; of course taking into consideration the existence or non-existence, form and structure of the shell, on the same principle, which, in arranging the vertebrated animals would lead us to study the hair, hoof, nails, claws, &c. as well as the other parts.
At the same time, it must be admitted that there are many private collectors of Shells who would find it a difficult, if not impossible task to study minutely and successfully the soft parts of the Mollusca. Ladies, for instance, could not be expected to handle with pleasure and perseverance, these fleshy substances, which in order to be preserved from putrefaction, must be kept in spirits; and yet such persons may, with improvement and advantage to their own minds, enjoy the interesting and scientific amusement of studying and arranging the clean and beautiful natural objects which are so easily preserved, and so exquisitely curious in their structure. Let it also be remembered, that if shells had not been rendered commercially valuable, by the zeal and emulation manifested by mere Conchologists for the possession of rare specimens, few travelling merchants and sea captains would have thought them worthy of a corner in their cabins. In this case, few specimens being brought to the country, the more Philosophical Naturalist would have been left without the means of obtaining materials to work upon, or of attracting public attention to his favourite pursuit.
On account of these and other considerations, it has been thought advisable that the present undertaking should bear a purely conchological character. The peculiarities of the shells alone being detailed for the assistance of those who collect and study them, while at the same time, in deciding upon their affinities and places, in the arrangement, it will be necessary to take advantage of the conclusion to which those have arrived, who have studied the animal in all its parts. And the conviction must be expressed, that if ever a complete Natural System shall be formed it will result from the labours of the last mentioned class of naturalists.
DEFINITION OF A SHELL.
Before entering minutely into the description of shells, it will be necessary to distinguish from the true testaceous Mollusca two kinds of animals which have formerly been associated with them. Of these, the first is the class of Crustacea, consisting of crabs, crayfish, &c. These differ from shell-fish, not only in structure and chemical composition, but also in the fact that the animal has jointed limbs, and that the substance of the flesh is inseparable from the hard external covering, which invests each particular joint as with a sheath; whereas the Molluscous animal is but partially attached to its shell, from which it possesses the power of partly withdrawing and returning. The second class is that to which the sea-urchin, or Echinus, belongs, of which there are many genera and species. The testaceous covering of Echini is composed of a number of small pieces, placed edge to edge, forming a more or less globular external covering to the flesh, which is supported in the centre by a number of bones leaning upon each other in a pyramidal form. The test is of a fibrous texture, guarded on the outside with moveable spines, which turn on ball and socket joints.
A true shell is composed of one or more calcareous pieces, commonly called valves, each piece formed by a series of layers, applied obliquely upon each other, in such a manner that each new layer begins within, and terminates a little in advance of the one before it.
STRUCTURE AND GROWTH.
We shall now endeavour to describe the manner in which the growth of each separate valve, or each regularly formed shell, proceeds from the nucleus.
Before the young animal has left the egg, if it be an oviparous species, or the body of the parent if viviparous, the nucleus of the shell is generally formed, and specimens are sometimes preserved in which the young shell is seen within the egg, as in the cut, fig. 1, 2; or adhering to the inner surface of the full-grown shell by the dried mucus of the animal, as seen in fig. 3.
1. Egg of a Bulinus. 2. The same broken, shewing the young shell. 3. The young of a Paludina, as seen in the aperture of the shell.
In both cases, the nucleus is generally of a more horny and transparent composition than the parts subsequently produced. As soon as the animal is hatched, or, in other words, leaves the egg or body of the parent, of course it begins to increase in size, and to require a corresponding enlargement in the shell. To effect this, a small quantity of mucus substance, secreted by the mantle of the animal, is deposited on the edge of the aperture. When this is dry and become sufficiently hard, it is lined by a more calcareous secretion; and these together form a new layer, which is followed by others in succession; each new layer being larger than the one that preceded it until the whole being complete, the full-grown animal is invested with a shell commensurate with its own proportions. Thus from the apex or nucleus the formation proceeds, as it were, downwards, taking the shape of the part which secretes it, on which it is in a manner moulded.
The nucleus, or first formed portion, may for technical purposes be considered, mathematically, as the apex of a spiral cone. And here it must be observed, that whether the shell consist of one or several pieces, each piece has a separate nucleus, and the process of formation is separately repeated with each. The word cone is used for convenience, and its meaning extended so as to include all those structures which commencing at a point enlarge downwards.
4. Imaginary cone. a. Apex. b. Base. l. Lines of growth.
From the apex, the next layer is deposited on its edge, and advancing beyond it necessarily adds to its extent. Thus, suppose for the sake of illustration, the part marked a in the diagram, fig. 4, to represent a nucleus, the cross lines (l) will shew the consecutive layers, which enlarge their circle as they add to their numbers. This disposition of shelly matter into layers is marked externally by concentric striæ, or lines of growth, while on the inside the edges of the laminæ are consolidated into a kind of enamel. If a perpendicular section of a solid portion of a shell were magnified, it would present, in many instances, an appearance resembling the diagram, fig. 5; a may be taken to represent the horny part of the layers which form the outer coating, named "Periostaca," or "Epidermis;" the undulating line b, is formed by the edges of the calcareous layers, and causes the striæ, or lines of growth, which are often distinguishable on the surface of the shell; the space c is the middle part of those layers, and at d they are consolidated into the enamel which lines the interior.
5. Supposed section of a part of a solid shell.
In some species the layers are irregularly grouped together, and their edges overlap each other, so that they are easily separable, and advancing beyond each other, give a leafy appearance to the external surface. This structure is termed foliaceous. A very familiar instance of this may be observed in the common oyster. If a specimen of this shell be broken, the substance will be seen to exhibit a degree of looseness, and a magnifying glass will enable the student to trace distinctly the laminæ of which it is composed. The accompanying representation of a magnified section (fig. 6) will shew at a, the external surface, with the foliations or leaves; at b, the parcels of layers which form them; and at c, the pearly structure produced by their consolidation, and by the subsequently deposited enamel which covers their external surface.
6. Section of an oyster shell enlarged.
CLASSIFICATION.
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.
Accessary valves of a Pholas.
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.
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.
13. Section of Nautilus.—14. Undulating Septa.—15. Arborescent Septa.
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.
Fig. 25, obtuse; 26, acute; 27, 28, decollated; 29, concave; 30, papillary; 31, mammellated; 32, discoidal.
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.
Fig. 33, few; 34, numerous; 35, rounded, ventricose; 36, angular, ventricose; 37, flattened.
Suture.
Fig. 38, canaliculated; 39, enamelled.
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.
Fig. 40, numerous; 41, few, continuous; 42, few, discontinuous.
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.
Fig. 43, Helicina, semilunar; 44, Pirena, pyriform; 45, Cypræa, linear; 46, Trochus, trigonal; 47, Cyclostoma, rounded; 48, Chilina, posteriorly contracted; 49, Stomatia, transversely oval; 50, Murex, longitudinally oval.
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.
When there is neither notch nor canal, anteriorly or posteriorly, interrupting the edge of the aperture, it is described as entire. When there is a notch or sinus at the anterior extremity, it is said to be emarginated. When the edge of this notch is expanded, and drawn out in the form of a beak, it is said to be canaliferous, or to have a canal. When, in addition to this, the lips are thickened and contracted posteriorly near their junction, and drawn out so as to form a groove, it is said to be bi-canaliculated, or to have two canals. The anterior canal is said to be long or short, according to the proportion which it may bear to the rest of the shell. Thus the canal of Ranella ranina (fig. 393 in the plates), may be described as short; while that of Murex haustellum, (fig. 396, pl.) is long. When it is wide near the aperture, and becomes gradually contracted towards its termination, it is said to be tapering, as in Pyrula (fig. 388, pl.); when the termination is sudden, it is described as truncated. If, on placing the shell upon a plane, with the aperture downwards, the canal is seen to rise upwards, it is recurved. In Buccinum and Nassa it is turned suddenly over the back, and forms a short, curved elevation; it is then described as recurved and varicose. If the edges meet, so as to form a tube, it is said to be closed, as in some species of Murex and Typhis. The posterior canal is, in some cases, free, or standing out from the spire, as in some species of Ranellæ; while in others it is decumbent, running up the sides of the spire, as in Rostellaria (fig. 402, pl.).
Canals.
Fig. 51, Fasciolaria, truncated; 52, Nassa, recurved, varicose; 53, Cerithium, recurved; 54, Typhis, closed.
Lips, or edges of the Aperture.
The part of the edge of the aperture next to the body whorl is named the inner, or columellar lip. Posteriorly it commences at the point of union with the outer lip, where that touches the body whorl, the junction being generally marked by an angle, and sometimes by a canal. Anteriorly it terminates where there is generally seen a notch or canal, or sudden angle, from which the outer lip proceeds. The part which setting out from the body whorl, and proceeds outwards at a distance from the axis, till it reaches the anterior canal or notch (or its place in case of absence) is named the outer lip. In many cases the edges are united in such a manner, that it is difficult to distinguish where the inner lip terminates, and the outer lip commences: when this is the case, it is usual to describe the margin or peritrême, as a whole, without distinguishing the parts. The outer lip, sometimes called the right lip, or labrum of continental writers, is sometimes acute, not being of thicker substance than the remainder of the shell. In other cases it is obtuse, or thickened and rounded at the edge. When thickened and turned backwards it is described as reflected; when, on the other hand, it is turned inwards towards the axis, as in the Cyprædæ, it is inflected, or involute. When it is toothed, a distinction must be observed as to whether the dentations are external or internal. If the teeth are small and numerous, it is denticulated; if larger, it is dentated; when expanded into a kind of wing, as in some species of Strombus and Rostellaria, it is described as alated; and a family in Lamarck's system is named "Alatæ," from this very circumstance. In some of those which are expanded, the expansion is divided into separate, attenuated portions, they are then said to be digitated.
Outer Lips.
Fig. 55, Helix, reflected; 56, Cypræa, involute, denticulated; 57, Sera, alated; 58, Murex, digitated; 59, Rostellaria, dentated.
The inner lip, sometimes named the columellar lip, or "labium," is subject to similar variations as to thickness, dentition, &c. That portion of it which lies upon the body-whorl is frequently distinguished from that which intervenes between it and the notch or canal. De Blainville, restricting the term lip "bord gauche" to the former portion, applies the term "columella" to the latter; and in some instances this may be the more convenient method of describing the part in question. The columellar lip is sometimes detached entirely from the body of the shell, as in Murex haustellum; in others it is decumbent, or lying over the last whorl, although quite distinct, and in some cases, thickened, callous, or tumid.
At the lower or anterior part, sometimes called the columella, there are in many instances flattened, laminated folds; these are particularly conspicuous in the genera Cymba and Melo, where, being obliquely spiral and laminar, they are extremely elegant, presenting to the eye graduated repetitions of the line of beauty. In other cases, as in the Turbinellæ, they are more horizontal and thickened.
In some cases the columella is swelled into a varicose mass; as in Oliva, Ancillaria, &c.; it is then described as tumid or varicose. It is sometimes tortuous, and sometimes straight, and is susceptible of many variations, too minute and particular to be described in this part of the work.
Columellar Lips.
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.
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.
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.
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.
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.
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 generally the case with shells which are attached to submarine substances, such as Spondyli, Oysters, &c.; and the degree of irregularity will depend upon the extent of surface involved in the attachment. In the case of fixed shells, the attached valve is usually termed the under valve, and the other which moves freely upon the hinge, is termed the upper valve.
Form and Proportions.
Bivalves are said to be equivalve when the two valves correspond in extent, breadth, and thickness; and of course inequivalve in the contrary case. They are equilateral when a line drawn from the umbones to the ventral margin would divide the shell into two nearly equal parts; and of course inequilateral in the opposite case, which occurs in the great majority of instances.
A Bivalve is said to be compressed, when the distance is small from the most prominent part of one valve to that of the other. It is cylindrical when lengthened, and more or less rounded in its breadth, as in Lithodomus (fig. 161 in the plates). It is cordiform when the shape presents a resemblance to an imaginary heart, as in Cardium cardissa (fig. 122 in the plates), and in the Isocardia (fig. 126 in the plates). It is linguiform when it resembles a tongue in shape, as in Vulsella (fig. 185 in the plates); rostrated when it protrudes at either extremity, and terminates in a kind of point, as in Sanguinolaria Diphos (fig. 99 in the plates); truncated when it ends in a square or angle, as if cut off; an example of which may be seen in Solen (fig. 60 in the plates).
Other Bivalves are distinguished as being auriculated, having processes flattened and expanded on either side of the umbones, as in Pecten (cut, fig. 82). When there is one of these on each side of the umbones, it is bi-auriculated; when only on one side, it is uni-auriculated. When the expansion is very broad, as in Unio alatus (fig. 142 in the plates), and in the Hammer Oyster (cut, fig. 83), the term alated is used.
Fig. 82, auriculated; 83, auriculated, alated.
With regard to these alated species of Uniones, it is necessary to observe that they are also "adnate," as it is termed; the two valves being joined to each other by the dorsal edge of the expanded parts, and united so completely in substance with each other, that they cannot be separated without being broken. Many other terms are used to express difference in Bivalves, but being generally applicable to Univalves and Multivalves, as well as to them, they will be found explained at large in the alphabetical part of the work.
MULTIVALVE SHELLS.
These are of three different kinds; first, the "dorsal," as they are termed by Linnæus, because they form a ridge in the back of the animal. They are composed of eight pieces, or separate valves, placed in a longitudinal series, being joined to each other by inserted lamina, and named Articulata by De Blainville, on that account. The genus Chiton is the only example of this kind of Multivalves.
Fig. 84, 85, Chiton. a, anterior; p, posterior; d, dorsal ridge; l l, lateral areas of the valves; c c, central areas; i i, inserted lamina; m, margin.
The second kind, M. De Blainville terms the lateral bivalves, the pieces being placed in pairs on each side of the animal; these compose the "Pedunculated Cirripedes."
Fig. 86, Anatina.
They differ considerably in the number and arrangement of the valves; the small ones, which are found near the peduncle in some species, are sometimes termed accessary valves; those which form the edge through which the bunch of Cilia protrude, are termed ventral, and those on the opposite side dorsal. The extremity joining the peduncle is the basal, or anterior; and the upper extremity is the apsiral, or posterior. The peduncle is the medium of attachment to submarine substances, to which this well known tribe of shells adhere.
The third kind are termed coronular by De Blainville, and compose the order Sessile Cirripedes of Lamarck; they consist of a number of valves placed against each other side by side in a circle, supported on a plate, or tube, or cup, and closed by an operculum composed of two or more valves.
The basal support is sometimes thick and flat, sometimes forming an elongated tube, and sometimes hollowed out into a cup. In other species it is altogether wanting. The operculum always consists of more than one piece, generally of two pairs: they are either articulated to each other by serrated edges, and placed against each other conically, as in Balanus, or they lie flat in two pairs against each other. Through the ventral pair the cirrhi protrude.
The parietal valves, composing the principal part of the shell, vary in number, form and position. The anterior valves are placed on the same side with the cirrhi; the posterior, those on the opposite side; and those which remain between on each side are the lateral valves. In many cases, particularly in Balanus, each valve is separated into the prominent and depressed areas, and the inserted lamina. In some instances, the parietal portion is formed by a single rounded piece.
In the accompanying cut (87), the prominent areas are distinguished by the letters pr, and the depressed areas by r; the posterior valves of the operculum are marked p. o., and the anterior a. o. The basal valve (fig. 88) belongs to a Balanus. Fig. 89 is an Acasta, the cup-shaped base of which is represented at fig. 90.
In the foregoing explanations we have omitted many of those general terms which, relating to external characters, are applicable to shells in almost every division of the system. It may be as well, however, to enumerate a few of them in this place, although they are explained under their respective letters in the alphabetical part of the work.
When bars or ribs, or large striæ are crossed by others radiating from the umbones, shells are said to be cancellated, as represented in cut, fig. 91. When there is a series of nodules or spines on the upper part of the whorls, they are coronated, as shewn in cut, fig. 92. When a series of projecting parts overlay each other, in the manner of tiles, as in the cut, fig. 93, the word imbricated is applied. When marked by a regular series of ridges, radiating from the apex, they are pectinated; the species of Chiton, a single valve of which is represented in cut, fig. 94, has received the specific name of pectinatus, in consequence of this character. Shells are said to be plicated when characterized by angular bendings or foldings in their surface, as shewn in cut, fig. 95. A strong instance of this is seen in the Ostræa Crista-Galli. When the margin of any shell has a series of minute notches, resembling the teeth of a saw, it is said to be serrated; when covered with raised points or spines it is aculeated; and when striated in both directions, it is decussated; when covered with a number of raised rounded points, it is granulated; and having a series of these points placed in a row, near or upon the edge, it is denticulated, as already explained in reference to the outer lips of Spiral Univalves. When the external surface is rendered uneven by raised knobs, it is said to be tuberculated; and if rendered rough and prickly by sharp points it is muricated, as in the cut, fig. 97. The term reticulated is applied to fine raised lines, crossing each other, and resembling fine net-work.
External surface. Fig. 91, cancellated; 92, coronated; 93, imbricated; 94, pectinated; 95, plicated; 96, decussated; 97, muricated; 98, foliated.
By the foregoing general observations and explanations, it is trusted that the reader will be prepared for the following exposition of the general arrangement of Lamarck, and the principles upon which it is founded.