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FmmSy Composition, and Structure of Shells, 345 



their shell ;" * and thus it happens, that, in cones, the " inner 

 concealed volutions " are ''exceedingly thin," while "the 

 outer or exposed parts are comparatively thick and strong." 

 So, likewise, the transverse spinous ridges which thicken and 

 protect the external lip of the Murices, or rock shells, are 

 removed, probably by a solvent liquor ; " for otherwise, he 

 adds, " it is obvious that these murications or spines mUst be 

 in the way of the future increase of the shells." Let us for a 

 moment admit this hypothesis, and trace the consequences. It 

 supposes the existence of a solvent liquor, and of a glandular 

 apparatus fitted for its secretion ; it supposes that this gland is 

 in action only at particular seasons, and then only for a very 

 limited time, after which it good*-naturedly remains quiescent ; 

 it supposes that the living flesh of the creature is insensible to 

 the burning of this acid, which, it further supposes, is so equally 

 and carefully applied, that the inner volutions are worn only 

 " exceedingly thin," but in no place perforated or destroyed ; 

 and so much only of the spinous lip of the ikfurex dissolved 

 that the spines fall off, and leave the part smooth and even, 

 a state in which an acid never left a part before. It supposes 

 (and this is a startling supposition) that the snail swallows the 

 redissolved lime charged with all its solvent liquor, which, it 

 would seem, passes through the body to reach the excreting 

 organs, and be again deposited. Or, if the supposition is not 

 to be admitted, by what organs, I ask, is the liquid lime pre- 

 vented from being washed away; and by what organs is it 

 purified of the acid, and again spread out and solidified ? A 

 hypothesis which requires so many and such strange supposi- 

 tions for its support cannot for a moment be entertained. It 

 ." is itself so exceedingly absurd," to use Mr. Sowerby's own 

 words, when combating the less unreasonable doctrine of a ce- 

 lebrated naturalist, " that we could have wished, for the credit 

 of science, that it had never disgraced the pages of any books 

 on natural history : " and if any other explanation than what 

 we have given is required to explain the facts, it must be 

 sought for in a more perfect knowledge of the structure of the 

 animals. 



Let me now direct your attention to the colouring of shells. 

 The shell is rarely coloured throughout. The interior layers 

 are commonly white ; the internal surface is sometimes par- 

 tially coloured, the external one almost always. 



The fine iridescence of the internal surface of many uni- 

 valves and bivalves is not owing to any colouring matter, but, 



* This remark seems to have been made, in the first instance, by Mr. 

 Burrows, in his Elements of Coiichology^ p. 95. The fact is very singular, 

 but I do not see exactly how it bears on the question at issue. 



Vol. IIL — No. 14. a a 



