86 Colouration in Animals aad Plants. 



stones of the edifice. The shell is built up from the edge, and the 

 action is not continuous but seasonal, hence arise the markings 

 known as lines of growth. In some cases the mantle is expanded 

 at times into wing-like processes, which are turned back over the 

 shell, and deposit additional layers, thus thickening the shell. 



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 (Cypraa), 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 Ave should expect. The Nautilus repeats 

 in the red-brown markings of its shell, the shape of the septa which 



