INTRODUCTORY CHAPTER. 21 



the new piece; but if the fracture has been 

 made at a distance from the aperture, and in the 

 body of the shell, a new portion will indeed be 

 made by that part of the mantle which has been 

 laid bare, but it will be white or nearly colour- 

 less ; thus proving, that, although every part 

 of the mantle can produce shell, the external 

 edge alone can infuse colour into it. The bril- 

 liancy of the colours observed in shells is ex- 

 ceedingly influenced by the degree of health, 

 and by the food, light, and heat which the 

 mollusc has had the power of enjoying. Those 

 which inhabit tropical countries have often a 

 very great degree of brilliancy and beauty ; 

 whilst others which are less favoured by cli- 

 mate, and, still more remarkably, those which 

 remain all their lives enclosed in wood, chalk, 

 &c, have little or no colour. It is observable, 

 also, in a similar manner, that a great differ- 

 ence frequently exists in the size of mature 

 shells of the same species, particularly in such 

 as have the strongly marked lip, or other final 

 appendage : this may be accounted for by the 

 strength or weakness of constitution existing in 

 c 3 



