SYSTEMATIC DISTRIBUTION. 
5 
in fact^ moulded on the body of the animal itself, as 
the body grows ; and for this reason any inequality 
in the body is moulded in the shell. 
The animal has the faculty, also, of mending any 
break or injury that its shell may have received, if it 
is not of such a magnitude as to derano;e all the func- 
tions of the animal itself ; and it mends them in the 
same manner as it forms its shell, that is to say, 
by depositing first a coat of animal matter, and then 
lining it with mucous matter, mixed with chalk to 
harden it. But as the animal is usually very desirous 
of getting the repairs done as quickly as possible, 
and is most probably damaged by the injury it has 
received, these repairs are generally much more 
roughly executed than the shell itself, and com- 
monly destitute of regular colour. 
The particles which vary the colour of the surface 
of the shell are deposited, while the shell is being 
increased in size, immediately under the outer mu- 
cous coat ; and as these particles are also secreted by 
peculiar glands, the colour is always situated in a 
particular manner on each species, the glands being 
gradually enlarged, and gradually separated, but 
not changed in position, by the growth of the animal. 
All the variations exhibited in the colouring of the 
different species, or in the different individuals of the 
same species, are produced by the permanent or 
temporary interruption of the action of these glands. 
But for a more detailed account of these phenomena, 
I must refer the reader to my papers on the subject 
in the Philosophical Transactions 1833, reprinted 
in Dr. J ohnston’s Letters on Conchology. 
