other body, although I have found, at the junctions of the adductor muscle with the shells, both in Pinna and Ostrea, a layer of most elaborate and complex minute vascular tissue, and amid these other vessels, few in number and much greater in size. These vessels were evidently not those which appertained only to the mussel itself, as they were found in no other part of it, but concentrated in a single complex stratum at this point.'

You observe, then, that Mr. Bowerbank concludes the shell to be highly organized and vascular, and that it retains, during life, a vascular communication with the animal it protects. It has a structure analogous to bone in some respects, and is formed much in the same manner by the deposition of carbonate of lime within the cells of the membranes of which shell is composed, or by the aggregation and coalescence of the calcigerous cells when the membrane is very sparingly produced. Let us go back to the Mollusk when yet within its egg, but far enough advanced to be +endued with the rudiment of its future shell. We may suppose this rudiment to have been the result of the excretion of some mucus or lymph, and it is in fact nothing more than a very thin transparent membrane, with a determinate figure dependent on the figure of its species. In this membrane organizing cytoblasts and cells are produced and multiplied in rapid succession until, by their increase and apposition, a cellular structure is formed in it. On their first appearance the cells are transparent and globular, but, pushed on by the law of growth, which regulates their development, they very soon begin to secrete, from their inner surfaces, carbonate of lime. The cells being filled with it, a solid structure is the result of their close packing and aggregation, and this structure must, of course, exhibit the cellular structure whence it has been derived, the pattern being modified only by the form and degree of condensation of the calcigerous cells in which it has been secreted. As this deposition of calcareous matter proceeds, canals are also formed, which penetrate the layer, and another system of vessels which are destined to maintain its life by leading through it a circulation of fluids. A layer or stratum of shell being thus formed, another is produced from its inner surface, by the same production of a basement membrane and the same development and aggregation of calcigerous cells; and then others until the normal number set for the species is completed, the whole being, however, kept together as one by the living tissues and vessels. Mr. Bowerbank thinks that the truth of this is proved not only by the structures he

has discovered in shell, but also by the phenomena which occur in its reparation of injuries, whether that injury is limited to the periostracum or inflicted on the substance of the shell itself; for this reparation is not made by a coat of calcareous matter spread over the wound by the collar or mantle of the animal, as has been maintained, but by an effusion of coagulable lymph in which cytoblasts are produced in the first instance, and quickly succeeded by a cellular structure, in which the earthy basis of the shell is secreted, and by which the scar is filled up, or the fracture cemented together.*

Dr. W. B. Carpenter entered simultaneously with Mr. Bowerbank on the investigation into the structure of shells, but as his objects were different, he was led to work after a more systematic fashion. Dr. Carpenter's principal object appears to have been to discover whether the microscopic. structure of the shell was so peculiar and distinctive that from it we might conclude as to its genus or family, so that henceforth the conchologist, from an inspection of even a fragment of a valve or whorl, might rival the zoologist in conjuring up, on fixed principles, the image and character of a beast of whose existence he is only made aware by a few fragmentary remains. And Dr. Carpenter has been eminently successful so far as the bivalve shells are concerned; for there is too much sameness in the structure of univalves to give the same satisfactory results. This eminent physiologist has ascertained that an uniform structure prevails throughout every part of a shell, and, consequently, that the examination of but a very small piece suffices to determine its entire structure; and he has also ascertained that the species of the same genus present essentially a oneness in pattern, so that any material deviation from it indicates a family or generic difference in the structure of the animal. The conchologist who formerly broke the shell that he was quarrying from its ancient tomb, lost his labour and the key to the knowledge it might have disclosed to him; but now, if the shell be a bivalve, he has only to prepare a fragment of his shattered document, and the key is recovered that locked up in secrecy the world's former tenantry. Such is one result of Dr. Carpenter's curious researches ; but in proving it he has necessarily discovered many important facts bearing on the subjects of this letter.

Dr. Carpenter concludes with Mr. Bowerbank, that all

* Observations on the Structure of the Shells of Molluscous and Conchiferous Animals, by J. S. Bowerbank, F.R.S., &c., published in the Transactions of the Microscopical Society, vol. i. 123. Lond. 1844.

the shells of molluscous animals possess organic structure, but he explains differently the appearances indicative of their vascularity, and denies their vascular connection with the animal. He bears witness to the correctness of Mr. Bowerbank's description of the structure of the univalves; but he discriminates with nicer precision the various structures exhibited by the bivalves. Of these beautiful organisms I could not give you an intelligible notion without the aid of the figures which illustrate Dr. Carpenter's Reports, and the attempt would be beside my purpose in this place, where I wish only to indicate the principal varieties—which are named the Cellular, the Membranous, and the Cancellated.

Of the cellular variety, the Pinna affords the most characteristic example. The shell is composed of a vast multitude of prisms, having for the most part a tolerably regular hexagonal shape and nearly uniform size. These are arranged perpendicularly, or nearly so, to the surface of each lamina, so that the thickness is formed by their length, and the two surfaces by their extremities. A "satisfactory view of these prisms is obtained by grinding down a lamina until it possesses a high degree of transparency, and it is then seen that the prisms themselves appear to be composed of a very homogeneous substance, but that they are separated by definite and strongly-marked lines of division. In general the substance forming the prisms is very transparent, but here and there is seen an isolated prism, usually of smaller size than the rest, which presents a very dark appearance, even in a section of no more than 1-400th of an inch in thickness, as if the prism contained an opaque substance." This opacity appears to be due to the presence of a small quantity of air in or near the extremities of the cells.

The membranous structure is the prevalent one, embracing all those shells that do not present the prismatic cellular tissue. In these the calcareous matter is deposited in laminæ separated by an excessively thin membrane, which forms, in fact, a secreting surface. No trace of cells can, for the most part, be discovered; and when they do present themselves, they are usually scattered through the substance with little or no regularity, and do not form a continuous stratum when the calcareous matter has been removed by an acid. "In no shell," says Dr. Carpenter, even those most decidedly porcellanous, have I failed in detecting some membranous basis, although the film is often of extreme tenuity. I believe that there is no shell, in which this kind of structure does not exist under some

form; for even where almost the entire thickness is made up of the prismatic substance, as in Pinna and its allies, there is still a thin lining of nacre, which I shall presently show to be but a simple modification of the ordinary membranous structure."

Of this membranous structure there are two kinds, the nacreous and the tubular. The nacreous or pearly is produced by the membranes being crimpled with numerous very delicate folds; and these most fine folds being repeated in a regular manner, give to the pleased eye the mother-ofpearl lustre which so brilliantly distinguishes the inner surface of many shells. The nacre or pearl is not then produced, as has been maintained, by the alternation of numerous layers of membrane and calcareous matter; but is due "to the plication or folding of a single layer, in such a mode that the folds shall lie over one another in an imbricated manner."

The other kind of membranous structure is the tubular. "All the different forms of membranous shell-structure are occasionally traversed by tubes, which seem to commence from the inner surface of the shell, and to be distributed in its several layers. These tubes vary in size from about the 1-20,000th to the 1-2000th of an inch; but their general diameter, in the shells in which they most abound, is about 1-4500th of an inch. The direction and distribution of these tubes are extremely various in different shells; in general, where they exist in considerable numbers, they form a network, which spreads itself out in each layer, nearly parallel to its surface; so that a large part of it comes into focus at the same time, in a section which passes in the plane of the lamina. From this network some branches proceed towards the nearer side of the section, as if to join the network of another layer; whilst others dip downwards, as if for a similar purpose. The most characteristic examples of this structure which I have met with are to be found in the outer yellow layer of Anomia ephippium, the external layer of Lima scabra, and in Chama florida. In other instances, the tubes run at a distance from each other obliquely through the shelly layers, and they are then usually of large size. This is the case, for instance, in Arca noæ and Pectunculus. In no cases have I seen any such variation in the ! size of the tubes of the same shell, as would convey the idea of their resemblance to blood vessels; and even where a division occurs, the size of each of the branches is usually equal to that of the single trunks. Sometimes these canals are quite straight, whilst in other instances they are sinuous.

!

That they are not mere channels or excavations in the shellsubstance is proved by the fact that they may be seen in the decalcified membrane. I have frequently seen in them indications of a cellular origin, as if they had been formed by the coalescence of a number of cells arranged in a linear direction; and I find that Mr. Bowerbank has come to the same conclusion.

"The tubular structure is usually found only in the ordinary membranous shell-substance; in fact, I have seldom observed it in the nacre, except where the tubes penetrate this, to be distributed in a layer external to it, as is the case, for example, in Anomia and Trigonia. I have nowhere found it coexisting in the same shell with any great amount of prismatic cellular substance; consequently it is for the most part absent in the Margaritacea and Nayadeæ, and but very slightly manifested in the true Ostraceæ. In most of the families of bivalves, however, in which the lobes of the mantle are united, some traces of it may be detected; though these are often very scanty. There is less regularity in regard to this character, than in respect to most others furnished by the microscopic examination of the shell. Thus I have found a little collection of tubes in one spot of the nacre of an Avicula, in no other part of which did I meet with any; and I have frequently found one species of a genus extremely tubular, whilst another, closely allied to it, was almost or entirely destitute of tubes."

The third kind, or cancellated structure, resembles the cancellated texture of bone, and is characteristic of a very peculiar group of shells, named the Rudistes.* Dr. Carpenter compares it to the prismatic cellular structure on a large scale, with this important difference, however, that the prismatic cells are not solid but hollow. "In what manner these minute chambers were occupied during the life of the animal, it is impossible now to say; as there is no existing group, to which the Rudistes seem to bear any close resemblance. The shape of each is usually that of a very short hexagonal prism, terminated at each end by a flat partition: consequently a section in one direction will exhibit the walls of the chambers disposed in a hexagonal network; whilst, when the section passes in the opposite direction, the transverse partitions come into view. The cancellated structure is externally and internally covered with a shelly plate, in which no perforations whatever can be seen. It

* This structure was first described by Mr. J. E. Gray in the Magazine of Zoology and Botany, ii. 228-32. 1838.