ordinary forms occurring in the artificial specimens, as, for instance, in spherical bodies and coalescing particles, so exactly like the artificial ones as to leave no doubt whatever of the identity of the process to which both the natural and artificial forms owe their origin. Such being the resemblance of the various parts of the shell in this class of animals, with the corresponding forms obtained by the artificial process, it now remains to examine the chemical conditions under which the natural products are formed, and to compare them with those of the artificial process.

Now, as these animals cannot be supposed to create or form de novo the carbonate of lime contained in their shells, either this substance or its chemical elements must be looked for in the medium they inhabit; but as it seems scarcely possible that any solid particles of carbonate of lime formed in this medium could penetrate the substance of the shell and arrive at the external surface of the deepest layer of membranethe part where the carbonate is first detected by the microscope-all its elements must of necessity be brought into this situation in a state of solution; and such can without any difficulty be shown to be the fact; as, on the one hand, in all crustaceans and molluscs, and without doubt all other animals having shells composed of carbonate of lime whose size is sufficient to admit of the application of the proper tests, carbonate of soda with albumen in solution can easily be detected on the surface of the animal next to the shell-(these are sufficiently abundant as at once to affect reddened litmus paper)—and the fluid from this situation deposits albumen on boiling; and on the other, there can be no difficulty in comprehending how the water, containing in solution different compounds of lime, can penetrate the passages of the shell above described, and diffusing itself through the entire shell, arrive at the superficial surface of the innermost membranous layer, and there, meeting with the sub-carbonate of soda and animal matter in solution, undergo the same decomposition with the same results as in the artificial process, or as was explained in describing the formation of the calculi found in the urine of the horse.

Now, if under such circumstances the different forms of the globular carbonate described in the shells of these crustaceans be the immediate result of an act of vitality, then will nature not only needlessly have brought together a set of conditions shown by experiment to be perfectly adequate to the same object, but she must also have been obliged to institute a separate act of interference between the salts of soda and lime, to prevent one decomposing the other. The production of an alkaline carbonate is a remarkable fact in the natural process, and results without doubt from the decomposition of some of the salts of potash or soda contained in the medium in which these animals live. Under what influence this decomposition is immediately effected is probably altogether unknown, and therefore it must remain classed with vital phenomena until a more advanced state of physiological science reveals the chemical, or perhap more probably the electrical, machinery which directly causes it.

The animals having an external skeleton, which come next in order,

are those comprised under the title of Mollusca. Of these I will describe such as are the most common, and in which the fact connected with the formation of their shells can be most easily verified by those who may wish to examine them for themselves. The shell of the muscle-and more especially of the oyster-furnishing all the necessary facts to be observed in this class of tissues, I shall confine my observations to them, taking it for granted that all the rest are formed after the same plan.

The shells of the muscle and oyster are laminated, like that of the crab or lobster, and grow in the same manner, by the successive additions of new layers to the internal surface of the layer last formed, so that each valve of a shell presents a cone, whose summit is the valve or layer first formed, and base, the layer which is formed last, the one in immediate contact with the surface of the mollusc. Each layer also consists of a membrane calcified, especially on its external surface. This arrangement is best demonstrated by putting an entire valve into hydrochloric acid, so feeble that the too sudden escape of the carbonie acid will not break the membrane into fragments, but raise it entire. The most external membrane of the shell of the muscle does not become calcified, nor does it receive an addition to its entire surface, but only grows at its free border. It is of a horny texture, and has distinctly cellular markings. But the internal membrane, as the muscle grows, receives an addition of new membrane to the whole of its inner surface. This membrane is perfectly homogeneous, presenting no markings whatever, excepting close to the edge of the valve, where it becomes continuous with the external membrane. Nearly the same arrangement exists in the lamina of the shell of the oyster, which, being less firmly adherent, are more favourably circumstanced for the examination of the process of calcification, the physical conditions favouring the deposit of globular carbonate in a globular form being less interfered with by the close proximity of the membranes on which it is deposited. Hence I shall confine my descriptions to the appear ances demonstrative of the nature of this process as they have occurred to me in examining the shell of the oyster. I may also observe that the very largest oysters that can be obtained-those which have large cavities between their lamina-are the best suited for this examination. I am not aware that the season of the year makes much difference in this respect, but season has doubtless some influence on the growth of these shells. The most successful examinations I have been able to make have been in the month of June. One way of exhibiting the structure of shells of this kind is by grinding vertical sections sufficiently thin to allow of being seen by transmitted light. This plan answers very well for showing their laminated character; but it gives no idea of their true structure, and much less of the manner in which they are formed. These can only be learned from the examination of extremely thin, partially-calcified laminæ, sufficiently transparent to allow of being examined by transmitted light without any previous manipulation which could injure their structure. The more incipient the calcification is, the more complete will be the spherical form of the

coalescing particles. These particles are best seen on those layers which are situated over the inter-laminar cavities before mentioned, between them and the internal surface of the shell. In such a situation the coalescing globular particles, being on a membrane extended between two spaces, will be very little impeded in their development by the mechanical pressure of contiguous structures; and hence the globular calculi so situated will be under circumstances at least as favourable for their coalescence into perfectly spherical figures as in the very imperfect shells of the smaller crustaceas, and almost as much so as in the artificial process. I have in my possession specimens of these bodies in every state of coalescence taken from such situations, so exactly like the artificial forms as not to admit of being distinguished from them by the microscope, even with the aid of polarized light. Partially calcified membranes, like the above, are most easily obtained by allowing oysters of the largest size to die for want of water, and the branchia to become dried to the shell, as in this state they can be most easily distinguished from the surrounding perfectly-formed shell, and also allow of being removed in larger films at one time. Such specimens are not to be found in all oysters, but only occasionally, to meet with just this stage of calcification being an accidental circumstance. The globular form of the carbonate of lime can at any time be seen, though in an inferior degree of completeness, on the outer surface of the septa, situated between the inter-laminar cavities. These cavities being always filled with water containing carbonate of soda and animal matter, this globular deposit must have resulted from the decomposition of the salts of lime before existing in this water, and the precipitation of the carbonate of lime combined with albumen upon the surface of these calcareous septa.

On the very thin septa found in these situations the exquisitely fine and perfectly homogeneous membrane on which the calcareous matter is deposited, can frequently be demonstrated without the aid of acid, more or less thickly covered with particles of globular carbonate in progress of transformation into plates of nacre; but I have never seen this membrane so folded as to decompose the rays of light, and thus produce a nacreous appearance, as has been supposed. I have never witnessed any such appearance where the calcareous matter has been completely removed, either mechanically or by an acid, which can only be determined with certainty by the polariscope: besides, the nacreous lustre of a piece of shell is not the least impaired by boiling it a long time in liquor potassæ, and but little so by heating it to redness, which must have been the case if produced by the delicate folds of what has been called the nacre-membrane. Now from what has been stated concerning the chemical part of the process by which the carbonate of lime in the globular form is produced and deposited in the shells of crustaceans, much does not remain to be said concerning the same process in these molluscs. The entire surface of animals of this class in contact with the shell is moistened with a solution of carbonate of soda and animal matter, which can be easily demonstrated, as before observed, by the proper tests, A piece of reddened litmus

paper, if allowed to remain in contact, for a minute or two, with the part of a muscle or oyster next the shell, never fails to show the presence of an alkali. From the same surface the membrane is exuded, on which the carbonate of lime is formed and deposited. This membrane does not follow exactly the surface of the shell, in consequence of the form of the oyster becoming more flat as its size increases, but bridges over the more concave parts of the shell, so as to leave a space or spaces between it and these parts, which, when the membrane becomes calcified and blended with the parts of this layer with which it was before in immediate contact, remain as the inter-laminar spaces before mentioned. These layers begin to adhere first about the middle of the shell, and become connected last at their borders, and thus there

is a free access of water containing the soluble salts of lime in solution to the outer surface of calcifying membrane, whilst, at the same time, the opposite surface being moistened by the alkaline solution furnished by the animal, the conditions necessary for the formation and deposition of the globular carbonate are the same as in the shells of crustaceans; and, as the optical characters of this natural product are the same as those of the artificial one, the physical agencies under which both are produced, may be inferred to be identical. Fig. 6 represents a vertical section of the shell of one of the largest kinds of oyster: aa, the inter-laminar spaces in which the alkaline solution was contained; bb, similar spaces, but filled up with amorphous carbonate of lime; c, septa between the spaces containing the fluid; B, portions of membrane, taken from the part where it bridges over the deepest parts of the shell, just beginning to calcify, showing this process in its diffe

rent stages; aa, the molecular state; bb, single spherules and dumbbell-shaped particles; c, laminated calculi, in which, in the specimen, a cross can be seen by polarized light.

Although the primitive and secondary forms assumed by the globular carbonate are alike in the crustaceans and molluscs, yet their ultimate forms are different in both; but as this difference depends upon mechanical causes, it can be imitated to a great extent in the artificial process, and thus shown to be the result of physical agency. In the crustaceans the ultimate form of the globular carbonate is the same as that of the artificial calculi, which particularly requires for their formation the removal of all such external sources of attraction as are capable of disturbing that of their own molecules. Such are the spherical luminated calculi. But in the oyster, as without doubt in all other molluscs, the ultimate form is that of flattened plates of nacre. This form can be produced in the artificial process by merely allowing the globular deposit to remain at the bottom of the vessel in which it was precipitated for several months. I have specimens taken from the shell of the oyster, in which this transformation can be seen in every possible stage, beginning with the form of regular spheres beautifully laminated, and going through every degree of flattening, the markings at the same time gradually disappearing, but last at the centre, until reduced to flat nacreous plates, generally with sharp irregular zig-zag edges. In this condition the plates show no tendency to coalesce. This form is produced conjointly by the pressure of the calcified membranes, between which the calculi are situated, and the attraction which these membranes exert on the calculi themselves. The latter is shown in the artificial process by the effect which the glass slide has in altering the form of the particles deposited upon its surface, and allowed to remain there a sufficiently long time. There is also another form called prismatic, which is situated chiefly about the edges of the shell. This is produced by the coalescence of molecules into globular bodies of different sizes, which occupying the softer parts of the shell, do not become pressed into plates, but which are still so crowded together as to be prevented from assuming the spherical figure. This form also has one analogous to it among the artificially prepared specimens. Thin sections of this form of natural carbonate, ground perfectly smooth and flat, present the appearance of compressed cells filled with air, having their convex sides looking upward-an appearance which, to the best of my knowledge, has never been correctly accounted for. It appears to me to arise from the fact of the rays by which this object is seen, not being those refracted by the flat ground surfaces, but by those molecules which are arranged in curves of which each piece is entirely made up, and of which the plane upper and lower surfaces of the section present arcs of different lengths. An appearance of the same deceptive character is presented by the siliceous cuticle of the common cane after having been boiled in nitric acid. This structure is made up of hexagonal pieces of silex, each having a flat upper and under surface, so joined together as to present one continuous plane, but each piece