678 



A HISTORY OF 



then another is added, which hardens also,, 

 till in time the shell becomes as thick as is 

 requisite for the animal's preservation. Thus 

 every shell may be considered as composed 

 of a number of layers of slime, which have 

 entirely proceeded from the animal's own 

 body. 



But though this be the general opinion 

 with regard to the formation of shells, 1 can- 

 not avoid thinking there are still other sub- 

 stances besides the .animal's own slime which 

 go to the composition of its shell, or at least 

 to its external coat, which is ever different 

 from the internal. The substances I mean are 

 the accidental concretions of earthy or sa- 

 line parts, which adhere to the slimy mat- 

 ter upon its first emission. By adopting this 

 theory, we can more satisfactorily account 

 for the various colours of the shell, which 

 cannot be supposed to take its tincture from 

 the animal's body, as is the usual opinion ; 

 for all the internal parts of the shell are but 

 of one white colour; it is only the outermost 

 layer of the shell that is so beautifully varied, 

 so richly tinctured with that variety of colours 

 we behold in the cabinets of the curious. If 

 the external coat be scaled off', as Mr. Angen- 

 ville asserts, all the inner substance will be 

 found but of one simple colouring; and con- 

 sequently the animal's own juices can give 

 enly one colour; whereas we see some shells 

 stained with a hundred. 



The usual way of accounting for the dif- 

 ferent colouring of shells, which seems to me 

 erroneous, is this: In th^> body of every one 

 of these animals, several- streaks are discern- 

 ed of a different colour from the rest. "This 

 variety," say they, " is an incontestable proof 

 that the juices flowing from those parts will 

 be also of a different hue; and will conse- 

 quently tinge that part of the shell which their 

 slime composes of a different colour." But 

 this system, as was observed before, is over- 

 thrown by the fact, which discovers that only 

 the outer surface of the shell is tinged ; where- 

 as by this it would have been coloured through- 

 out ; nay, by this system, the internal parts 

 of the shell would he stained with the most 

 vivid colouring, as being least exposed to the 

 external injuries of the element where it is 

 placed. But the truth is, the animal residing 

 in the shell has none of these various colours 



thus talked of: its slime is a simple, pellucid 

 substance; and (he only marblings which ap- 

 pear in its body, are the colour of the food 

 which is seen through its transparent intes- 

 tines. We must, therefore, account for the 

 various colouring of its shell upon a different 

 principle. 



if, as I said, we examine-the cabinets of the 

 curious, we shall find shells with Carious and 

 beautiful colouring; we shall find them ge- 

 nerally furnished with a while ground, tinc- 

 tured with red, yellow, brown, green, and se- 

 veral other shades and lovely mixtures, but 

 never blue. Shells are of almost all colours 

 but blue. The reason seems to be obvious; 

 for blue is the colour which sea-water changes. 

 A piece of silk, or a feather, of this colour, 

 put into an infusion of salt, urine, or nitre, 

 lose their tint entirely. Now, may not this 

 give us a hint with respect to the operation 

 of nature in colouring her shells? May we not 

 from hence conclude, that sea-water is effi- 

 cacious in giving colour or taking it away? 

 That, to produce colour, the animal not only 

 furnishes its juices, but the sea or the earth 

 that mixture of substance which is to unite 

 with them. Neither the animal slime alone, 

 nor the external earthy or saline substances 

 alone, could produce colours; but both united, 

 produce an effect which neither separately 

 were possessed of. Thus shells assume every 

 colour but blue; and that sea-water, instead 

 of producing, would be apt to destroy. 



From hence, therefore, it appears, that the 

 animal does not alone tincture its own shell; 

 but that external causes co-operate in contri- 

 buting to its beauty. It is probable, that, 

 from the nature of its food, or from other cir- 

 cumstances unknown to us, the external lay- 

 ers of its slime may be of different consisten- 

 ces; so as, when joined with the particles of 

 earth or salt that are accidentally united with 

 them from without, they assume various and 

 beautiful hues. But the internal layers, which 

 receive no foreign admixture, still pi^erve 

 the natural colour of the animal, and continue 

 white without any variation. 



Thus far we see that the animal is not whol- 

 ly the agent in giving beauty and colouring 

 to its shell : but it seems otherwise with re- 

 spect to its convolutions, its prominences, and 

 general form. These entirely depend upon 



