forms, 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 comparativ vely 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 sensible to 
the burning of this acid, which, it further swpposes, is so equally 
and carefully applied, that the inner volutions are worn only 
66 exceedingly thin,” but in no place perforated or destroyed ; 
and so much only of the spinous lip of the Murex dissolved 
ihat 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 w 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 entert ‘tained. 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 ina 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 Conchology, p.95. The fact is very singular, 
but I do not see exactly how it bears on the question at issue. 
Vou, Fil: — No, 14. AA 
