162 SUPPLEMENT 
it is a true colouring of impregnation, which appears to pro- 
ceed from the depurating organ. 
As for the nacreous or iridescent coloration which is still 
more frequently remarked at the interior of univalve and bi- 
valve shells, the experiments of Mr. Brewster (of which more 
hereafter) put it out of doubt that it is owing to the mechanical 
disposition of the molecules, and not to a really colouring 
matter. . 
The coloration of the external surface of the shells is alto- 
gether different, and in reality does not belong to them. It is 
always extremely superficial, and produced by the coloured 
pigmentum of the edge of the skin. These are the coloured 
molecules which are deposited above the calcareous deposi- 
tion, and which are of another nature, since they disappear in 
the course of time and by the action of heat. Accordingly 
the colour is so much the more lively, as the animal is 
younger, and as the produced part of the shell is more new. 
We are indebted to Reaumur for experiments which prove 
that it is only the anterior border of the mantle, which thus 
produces coloured molecules. In effect, it is quite certain 
that the new piece which is formed to fill a hole made in 
another part of the shell than its edge, is constantly white. 
We see, besides, that in the helix nemoralis, on which he 
made his experiments, and whose robe is agreeably zoned with 
black on a yellow ground, the part of the collar which corres- 
ponds to the black zones presents a tint of this colour, so that 
if we break a portion of the edge of the shell the piece which 
is reproduced is black opposite the black part of the border 
of the mantle, and yellowish on the rest. Although we have 
no direct proofs that such is the case with all the other shells 
which are coloured by zones decurrent from the summit to 
the base, analogy permits us to conclude that it is likely to 
be so; it must be confessed, however, that in the species 
