XXVI INTRODUCTION. 



tints, and sometimes a different disposition in their 

 symmetry, according to the degree of depth at which 

 they are situated in the sea. Bruguiere is led to 

 believe, that on this circumstance alone depends 

 the weakness of the tints with which a shell is 

 variegated ; and he even supposes that their total 

 privation is owing to the shell inhabiting a depth too 

 great for the due admission of light and heat. To 

 strengthen his assertion, he observes, that the bi- 

 valve shells, which fix themselves to the rocks, such 

 as the oysters, and spondyles, have generally their 

 upper valve deeply coloured, while the lower valve, 

 attached to the rock, is almost always white. The 

 oysters and spondyles are not the only shells that 

 show a constant difference in the colour of their 

 valves ; the same effect happens to all that live 

 fixed to any solid body. The pectens also, though 

 painted with various colours above, are white 

 beneath. The pholades and ship-borers, which 

 bury themselves in woods and limestone, are also 

 colourless. One position of this theory will not be 

 disputed, since there is no doubt that bodies wholly 

 deprived of light will be without colour ; but it still 

 remains to be proved, whether the tints of shells will 

 be much injured by a partial privation. 



