41 



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 

 of the [mantle-membrane] : the former we have 

 found to be more or less covered by glands ap- 

 pointed for the purpose, situated in the circum- 

 ference of the mantle ; but as these glands do not 

 exist elsewhere, no colouring matter is ever mixed 

 with the layers that increase the thickness of the 

 shell, so that the latter always remain of a delicate 

 white hue, and form the well-known iridescent 

 material usually distinguished by the name of 

 nacre, or mother of pearl. 33 * 



When we have removed the mantle-leaves, we 

 find beneath them the gill-leaves, two on each side. 

 These, like the mantle-leaves and the shell- valves, 



* Gen. Outline, 385. 



