320 ORDER III. TESTACEA. 



from some extraneous causes. It is well known that all 

 the internal parts of shells are of an uniform white colour ; 

 and that the outermost layer alone is so beautifully varied, 

 and so richly tinctured with vivid colours. Hence it ap- 

 pears that there must be an accretion of earthy or saline 

 matter arising from food or situation, exclusive of the 

 slimy fluid derived from the animal's own body, in order 

 to produce those pleasing tints which we witness in 

 cabinets of shells collected by the curious. 



In those repositories, which are sometimes formed at an 

 astonishing expense, are found shells possessing every shade 

 of colouring but blue ; the reason of which exception is, 

 the sea-water immediately annihilates that colour, while 

 it suffers others to exist. Put a piece of blue silk, or a 

 feather of this colour, into an infusion of salt, urine, or 

 nitre, and its tint will soon be extinguished. This may 

 throw some light on the operations of nature in regard to 

 shells. Neither the animal-slime alone, nor the external 

 earthy, nor saline particles individually, could produce 

 colours, but both united may yield an effect which neither 

 singly possessed. Hence it may be inferred, that the 

 animal alone does not tincture its shell, but that external 

 causes co-operate in adding to its beauty. 



But, though the shell with its beautiful tints is not pro- 

 duced by the sole agency of the animal, the fact is other- 

 wise with regard to its convolutions, its prominences, and 

 its general forms. These entirely depend on the art of 

 the animal ; or rather on the instincts, which in the same 

 species are ever invariable. The shell, indeed, always 

 bears some rude resemblance to the body on which it has 

 been moulded : wherever there is an excrescence on the 

 body, there is a prominence on the shell ; and a depression 

 is always accompanied with a correspondent hollow. li 

 the body is channelled, the shell that covers it will be 

 channelled likewise ; if annulated, the shell will wind 

 about it in the same spiral manner. 



Thus shells are as various in their figures as the animals 

 which they inclose. Indeed, the diversity of shells i? so 



