40 CONCHIFERl. 



matter, and deposits it in a soft state upon the 

 extreme edge of the shell, where the secretion 

 hardens and becomes converted into a layer of solid 

 testaceous substance. At intervals this process is 

 repeated, and every newly-formed layer enlarges 

 the diameter of the valve. The concentric strata 

 thus deposited remain distinguishable externally, 

 and thus the lines of growth marking the progres- 

 sive increase of size may easily be traced. 



" It appears that at certain times the deposition 

 of calcareous substance from the fringed circum- 

 ference 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. 



" 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 character, 

 and to owe their peculiar colours to a pigment 

 secreted by themselves : the pigment so furnished 

 being therefore mixed up with the calcareous mat- 

 ter at the time of its deposition, coloured lines are 



