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 mantis 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, are 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 



