16 MOLLUSCA. 
attain, the external surface is thus exclusively com- 
posed of layers deposited in succession by the 
margin of the mantle, and, seeing that this is the * 
case, nothing is more easy than to understand how 
the colours seen upon the exterior of the shell are 
deposited, and assume that definite arrangement 
characteristic of the species. We have already 
said that the border of the mantle contains, in its 
substance, coloured spots: these, when minutely 
examined, are found to be of a glandular character, 
and to owe their peculiar colours to a pigment se- 
creted by themselves; the pigment so furnished 
being therefore mixed wp with the calcareous mat- 
ter at the time of its deposition, coloured lines are 
found upon the exterior of the shell wherever 
these glandular organs exist. If the deposition of 
colour from the glands be kept up without remis- 
sion during the enlargement of the shell, the lines 
upon its surface are continuous and unbroken ; but 
if the pigment be furnished only at intervals, spots 
or coloured patches of regular form, and gradually 
increasing in size with the growth of the mantle, 
recur in a longitudinal series wherever the paint- 
secreting glands are met with. .... 
“While the margin of the mantle is thus the 
sole agent in enlarging the circumference of the 
shell, its growth in thickness is accomplished by a 
secretion of a kind of calcareous varnish, derived 
from the external surface of the mantle generally ; 
which, being deposited layer by layer over the 
whole interior of the previously existing shell, pro- 
gressively adds to its weight and solidity. There 
is, moreover, a remarkable difference between the 
character of the material secreted by the marginal 
fringe, and that furnished by the general surface 
