is very thick in this species, from which the principal

FIG. 182.

A vertical section of the

shell of the Pearl Oyster,

Avicula margaritifera, showing the prismatic and nacreous layers.

part of the Mother-of-pearl of commerce is procured. Horizontal sections of the shell of Lima scabra are chiefly composed of the subnacreous tissue, traversed by large branching tubuli not arranged in any definite order, but crossing and re-crossing each other in every direction, as shown in Fig. 183. When the shell is decalcified,

the tubes are still perfectly apparent, together with

FIG. 183.

Portion of the subna

the rest of the organic matter. The office these tubes perform is still uncertain, most of them commence upon the inner surface of the shell, which must have been in contact with the mantle, and then pass towards the outer surface, their function, as I have said before, may probably be analo

creous tissue of Lima scabra gous to that of the tubes of dentine; they are generally found only in

traversed by tubes.

the subnacreous substance, but the beautiful nacre of the Haliotis is traversed by them, as shown in Fig. 197.

LECTURE XVIII.

SKELETON OF MOLLUSCA-CONCHIFERA.

IN a perfectly-formed Oyster-shell there is always a very perceptible difference between the structure of the outer and inner lamina; the one is comparatively thick and soft, the other exceedingly thin and hard. In an old and thick shell which has been divided vertically, the two portions, as shown in Fig. 180, are very evident, the former being more or less calcareous, the other very thin. The calcareous portion is soft, containing scarcely a trace of animal matter, and is to a considerable extent destroyed by boring animals ; but they have left the thin, internal layers untouched, probably in consequence of their hardness. In some parts of such a section, nearly the whole of the calcareous external layer has been removed, and the laminæ composing the inner layer alone remain, so

ance.

that the shell in these parts has a cancellated appearA distinct cancellated structure is present in most of the shells of the genus Spondylus, the office of the cancelli being to render the shell lighter; the shell of Spondylus gigas is in some parts three inches thick, and if solid, would be very weighty, but if all the laminæ forming the cancelli were in close apposition they would form a stratum not much more than half an inch in thickness. It may be questioned whether the part corresponding to the outer layer, or that which in the Oyster filled up the spaces between the laminæ, ever exists in the Spondylus, or whether the laminæ, as we now see them, are really composed of two distinct kinds of shell-structure; this I shall endeavour to elucidate on another occasion.

The outer or calcareous laminæ of one of these thick oyster-shells, when ground as thin as possible, is still very opaque, except in some parts where the remains of prismatic structure may still be discovered. In the Pearl Oyster, Avicula margaritifera, the outer layer is generally very thin and coloured, the inner, thick and nacreous, and the flat valve is the upper one, as in the common Oyster. In a vertical section of this shell, shown in Fig. 182, the outer layer, which is of a greenish-brown colour, is composed of rows of prisms, whilst the whole of the inner layer is traversed by lines of growth; but when a portion of the same nacre is viewed under a much higher power, the sinuous structure is very apparent, and it is by such an arrangement

that the colours of this beautiful substance are produced. When the whole of the calcareous matter of the nacreous lamina of the same shell is removed by an acid, the organic tissue is found to be a closely plicated membrane; these folds are closer and more numerous where the colours are most brilliant, but, on the other hand, where no folds exist, the colours are entirely absent. The inner layer of the common Oystershell exhibits no such colours, and when decalcified, the membranous residuum is so small in quantity as hardly ⚫ to hold together. Many of the Mussels, as the Iridina elongata, have a nacreous internal layer, perhaps more brilliant than that of the Oysters, but these are all foreign species; the only British specimen at all resembling them is the Unio margaritiferus found in the rivers of Scotland, Ireland and Wales, which will be alluded to in a future lecture.

When Nacre or Mother-of-pearl was first examined, many years ago, by Sir David Brewster, he discovered on its surface a series of sinuous lines, which he imagined to be grooves, and they were compared by Sir John Herschell to the lines observed on a smoothlyplaned piece of deal; they are not grooves, however, but plications of the animal basis of the shell. The same effect is producible by grooves, as was first shown by Sir John Barton on the principle of unequal reflection of light; but I shall reserve the consideration of this subject for a future occasion.

In all the shells I have now described of the Lamelli

branchiate order, the lobes of the mantle are separate, and we have seen that the outer layer of the shell presents the prismatic cellular tissue, and the inner the nacreous or subnacreous. We have next to consider the structure of those shells in which the lobes of the mantle are either partially or completely united. In these shells the prismatic cellular tissue is very uncommon and the internal layer is generally subnacreous, sometimes presenting almost a crystalline ⚫ appearance, and when decalcified, the organic basis is so small in quantity, that a fragment of the section will hardly retain its shape when deprived of its calcareous

matter.

FIG. 184.

Many of the shells we are about to examine are provided with a horny epidermis, which is sometimes composed of cells like those of cuticle. The epidermis of our common edible Mussel contains traces of cellular tissue; Dr. Carpenter, however, believes that the epidermis of this shell is structureless, and that a layer immediately below it, which is cellular, is sometimes removed with it. In the specimen represented by A portion of the horny epidermis of Fig. 184, some of the cells had dark sel, Mytilus edulis. walls, others were black in the centre with transparent walls; these last were probably filled with air, the preparation being mounted in balsam. The periostracum of another shell, Trigonia Lamarckii, is undoubtedly cellular, each cell being hexagonal, with

the Common Mus