SHELL-FISH. 
243 
tive position with those that decorate the exterior of the 
shell. 
“When the animal is engaged in increasing the dimen- 
sions of its abode, the margin of the mantle is protruded, 
and firmly adherent all round to the circumference of the 
valve with which it corresponds. Thus circumstanced, it 
secretes calcareous matter, and deposits it upon the ex- 
treme edge of the shell, where the secretion hardens and 
becomes converted into a layer of solid testaceous sub- 
stance. At intervals this process is repeated, and every 
newly-formed layer enlarges the diameter of the valve. 
The concentric strata thus deposited remain distinguish- 
able externally, and thus the lines of growth marking the 
progressive increase of size may easily be traced. 
“ It appears that at certain times the deposition of cal- 
careous 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 period shoots out beyond its usual 
position, broad plates of shell, or spines of different 
lengths, aro secreted, which, remaining permanent, indi- 
cate, by the interspaces separating successively-deposited 
growths of this description, the periodical stimulus to 
increased action that caused their formation. 
“ 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. The border of the mantle 
