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. [Illustration] CHAPTER XIV. COLOURATION OF VERTEBRATA. The vertebrata, as their name implies, are distinguished by the possession of an internal skeleton, of which the backbone is the most essential part, and the general, but not universal, possession of limbs or appendages.