500 CONCHIFEEA. 



(1315.) It appears that at certain times the deposition of calcareous 

 substance from the fringed circumference of the mantle is much more 

 abundant than at others : in this case ridges are formed at distinct 

 intervals ; or, if the border of the mantle at such periods shoots out 

 beyond its usual position, broad plates of shell, or spines of different 

 lengths, are secreted, which, remaining permanent, indicate, by the 

 interspaces separating successively-deposited growths of this description, 

 the periodical stimulus to increased action that caused their formation. 



(1316.) Whatever thickness the shell may subsequently attain, the 

 external surface is thus exclusively composed 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 cha- 

 racter, and to owe their peculiar colours to a pigment secreted by them- 

 selves. The pigment so furnished being therefore mixed up with the 

 calcareous matter at the time of its deposition, coloured lines are formed 

 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 in- 

 tervals, spots or coloured patches of regular form, and gradually in- 

 creasing in size with the growth of the mantle, recur in a longitudinal 

 series wherever the paint-secreting glands are met with. 



(1317.) The carbonate of lime (for such is the earth whereof the 

 shells of bivalves are principally composed) is, at the moment of its de- 

 position, imbedded in a viscid secretion that forms a kind of cement ; 

 and on dissolving the shell in a dilute acid, the animal material thus 

 produced remains in the shape of a delicate cellulosity, in the interstices 

 of which the chalky particles had been entangled. If the proportion of 

 the above-mentioned secretion be abundant, it not unfrequently, by 

 hardening on the exterior of the shell, constitutes what has been very 

 inaptly termed its epidermis, representing a comparatively soft external 

 skin of semicorneous texture. If exceedingly thick, the epidermic 

 layer thus formed becomes loose and shaggy, giving the shell a hirsute 

 appearance ; but, both in its structure and origin, such pilose invest- 

 ment has no claim to be considered analogous to the hair of animals 

 possessing an epidermis properly so called. 



(1318.) While the margin of the mantle is thus the sole agent in 

 enlarging the circumference of the shell, its growth in thickness is accom- 

 plished 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- 



