ON MOLLUSCA. 163 
whose coloration is by oval, square, or irregular spots, and 
especially by transverse bands in the direction of the striz of 
augmentation, this analogy becomes less evident, unless we 
admit with Bruguiéres, that there is a change, a displace- 
ment, irregularly or not, in the parts of the border of the 
mantle which produce the coloured deposition, phenomena of 
which itis much more difficult to give an explanation, and 
which it would be necessary to submit to new observations. 
We have said just now that the coloration of the shells is 
constantly superficial; there is, however, one group in which, 
notwithstanding the existence of this superficial colouring, 
there is another deeper and not visible, and always very 
different not only in its kind but in its form. This group 
consists of the porcelains and some olives. Bruguiéres has 
perfectly explained this fact. During the course of a pretty 
long life these animals are invested, as we have seen above, 
with a very thin shell, its edges not denticulated, the spire 
visible, &c., and which is especially coloured at its superficies, 
as are the majority of shells. This coloration, owing to the 
edges of the mantle, takes place by degrees, along with the 
growth of the shell; but at a later period, perhaps when the 
animal is adult, the cutaneous appendages, which from each 
side of the body rise over the back of the shell, as ‘the animal 
crawls, deposit a calcareous ebony-coloured matter, which 
thickens by degrees, and at the same time a colouring matter, 
which constantly presents a disposition totally different from 
that of the first. We must then admit that the upper face of 
these cutaneous lobes presents spaces where the pigment is 
produced which colours the cutaneous matter thence exhaled; 
and, as in the development of these lobes, it is rare that 
these spaces fall precisely in the places of the first deposi- 
tions, we may conceive how this new coloration not only 
never takes place by decurrent bands, but is always exhibited 
in rather irregular spots. 
M 2 
