386 CONCHIFERA. 



concentric strata thus deposited remain distinguishable externally, 

 and thus the lines of growth marking the progressive increase of 

 size may easily be traced (fig> 179). 



It appears that at certain times the deposition of calcareous sub- 

 stance 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 differ- 

 ent 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. 



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

 the external surface is thus exclusively composed of layers de- 

 posited 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 secreted by themselves ; the pigment so fur- 

 nished 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 deposi- 

 tion of colour from the glands be kept up without remission during 

 the enlargement of the shell, the lines upon its surface are continu- 

 ous 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. 



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

 shells of bivalves are principally composed, is, at the moment of 

 its deposition, embedded 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 cellu- 

 losity, 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 



