In all the forms of life hitherto considered the colouring matter is deposited, or formed, in the substance of the organ, or epidermal covering, but in the mollusca this is not the case. The colouring matter is entirely upon the surface, and is, as it were, stencilled on to the colourless shell. This is precisely analogous to the colouring of the shells of birds' eggs. They, too, are calcareous envelopes, and the colouring matter is applied to the outside, as anyone can see by rubbing a coloured egg. In some eggs several layers of colouring matter are superimposed.
In no case does the external decoration of molluscan shells follow the structure lines of the animal, but it does follow the shape of the mantle. The secreting edge may be smooth, as in Mactra, regularly puckered, as in most Pectens, puckered at certain points, as in Trigonia, or thrown into long folds, as in Spondylus. In each of these cases the shell naturally takes the form of the mantle. It is smooth in Mactra, regularly ribbed in Pecten, tubercled in Trigonia, and spined in Spondylus. Where the inside of the shell is coloured as in some Pectens, regional decoration at once appears and the paleal lines, and muscular impressions are bounded or mapped out with colour.
It is a significant fact that smooth bivalves are not so ornate as rugose ones, and that the ridges, spines, and tubercles of the latter are the seats of the most prominent colour.
Similar remarks apply to univalve shells, which are wound on an imaginary vertical axis. They may be smooth, as in Conus and Oliva, rugose, as in Cerithium, or spined, as in Murex. The structure of these shells being more complex than that of bivalves, we find, as a rule, they are more lavishly ornamented, and the prominent parts of the shell, and especially the borders, are the seat of strongest colour. In some cases, as in adult Cowries (Cypræa), the mantle is reflexed so as to meet along the median line, where we see the darkest colour.
The rule amongst spiral shells is to possess spiral and marginal decoration, and this is what we should expect. The Nautilus repeats in the red-brown markings of its shell, the shape of the septa which divide the chambers, though, as is often the case, they are generally more numerous than the septa.
The naked Cephalopoda, or cuttle-fishes, often possess a distinct dorsal stripe, and when our views were first brought before the Zoological Society, this fact was cited as an objection. To us it seems one of the strongest of favourable cases, for these animals possess a sort of backbone—the well-known cuttle-bone—and hence they have a dorsal line.
Some shells, as Margarita catenata, have a chain-pattern, and in this case the action of the pigment cells takes place at regular and short intervals. Others, as Mactra stultorum, the stencilling forms a series of lines and spots, generally enlarging into rays.
The whole subject of the decoration of shells deserves much more time than we have been able to give to it as yet.
