ON MOLLUSCA. 161 
It is with this matter that the holes are filled, which 
accident may have made in the shell. The posterior part 
of the spire of those which are turriculated is also thus 
filled, which forces the animal to abandon it, and even the 
tubes or calcareous tunnels, which are formed by certain 
acephalous bivalve mollusca at particular periods of their 
life, are also filled in the same manner. In fine, it is by this 
vitreous deposited substance that the aperture of a tolerable 
number of univalve shells is narrowed, and that it often 
assumes quite another form than that which it had before the 
adult age of the animal. 
This part of the shell of the mollusca is so far remarkable 
that it is very brittle in all directions, somewhat after the 
manner of glass; this explains what is named by naturalists, 
the decollation of the spire in many cephalous mollusca. 
It is very rarely that the shell is coloured in its composing 
strata; in fact, in the far greater number of cases it is white: 
but, on the contrary, it is sometimes coloured in some parts 
of its internal surface, and almost always at the exterior. 
Every shell which is completely dermal is never coloured, 
which may be easily supposed, as the pigment remains at that 
part of the skin which covers it. 
The coloration which is sometimes remarked at the internal 
face, which seldom occurs, as would appear, except in the 
bivalves, belongs to the matter of deposition, and appears to 
be produced by an impregnation which extends by almost 
imperceptible degrees in surface and in depth. It is therefore 
probable that it is owing to some humour of the animal, pro- 
duced in an organ whose contact with the shell tinges it 
with the colour of this humour. This at least appears cer- 
tain as to the yellow or brown colour which is sometimes seen 
in the univalve shells. It is unquestionably owing to the 
contact of the liver. That of the janthina is in the same case; 
VOL. XII. M 
