80 
NATURAL HISTORY. [NEW BUILDING. 
the edge of the mouth of the shell; while those produced 
by the alternations of the periods of rest and developement 
are always in concentric lines parallel with the edge of its 
mouth. This method of formation and enlargement allows 
only the inner surface and the edge of the mouth of the 
shell, which is immediately applied to the surface of the 
animal, to be within its influence, as each deposition of 
new matter removes the part which it covers from the ani¬ 
mal’s control; consequently all the shelly appendages, 
&c., on the surface, except those on the immediate 
edge, which were used by the animal to protect its fleshy 
expansions, are no longer of any use to it, though they 
often add greatly to the beauty of the shell. 
The animal has the faculty also of mending any break 
or injury that its shell may receive, if it is not of such a 
magnitude as to derange all the functions 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 ani¬ 
mal matter, which is moulded on the body until it is dry, 
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 the shell has received, 
and also wants the support that the already formed shell 
gives to it during its growth, these repairs are generally 
irregular, much more roughly executed than the shell 
itself, and commonly destitute of regular colour. (See 
Cases 3 and 4, of the 5th Room of the Northern Zoolo¬ 
gical Gallery, and p. 160.) 
The shell of the unhatched animal is generally of a self 
or uniform colour, but after the animal is hatched, thesurface 
becomes more or less varied or ornamented. The particles 
which produce this colouring of the surface are deposited 
while the shell is increasing in size, immediately un¬ 
der the outer mucous coat, ( periostraca ,) and as these par¬ 
ticles are only secreted by peculiar glands, the colour is 
always disposed in a particular manner in each of the spe¬ 
cies, 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 different position, or 
