44> Experiments on the Growth of Pearls , 
are more liable to cracks and flaws than the coarser and more 
heterogeneous pearls, which are less glassy and more tenacious, 
as containing apparently more of the animal gluten. Nor is it 
to be wondered at, that those shells which contain a large pro- 
portion of their bulk of colouring matter, should have their pearls 
for the most part coloured. The pearls of the Pinna marina, 
when broken, are found to be equally radiated with the others, 
though their radiated structure is finer in proportion as they 
are more transparent. 
In the British Museum, there is, or was, a famous pink pearl, 
of a respectable size, and of an oval form : I myself have a small 
black pearl, perfectly round, and perfectly opake, of which I do 
not know the history. The pink pearl of the British Museum 
was probably produced by one of the large West India conchs. 
I have been told, that, in the waters in Scotland between Perth 
and Auchtermuchty, the mussels afford green pearls. The 
blue pearls from Montrose are also radiated in their texture, 
though extremely dense, so that the radii are invisible to the 
naked eye. 
I noted a particular appearance on breaking a red pearl be- 
tween two and three lines diameter, namely, that the great trans- 
parency of its external coats to the depth of one line, made its 
central sphere, which was less transparent, appear to the eye, be- 
fore the pearl was broken, as if a whitish opake sphere had been 
immediately placed beneath a thin external and very transparent 
coat ; and the same thing I have observed in another red pearl, 
where the opake central body was extremely small indeed, though 
perfectly visible, before breaking the pearl, and from the same 
cause. 
The structure of the greater part of shells, if not of all, con- 
sists of fibres parallel to each other, but perpendicular to the la- 
minae of the shell. We see this not only in the recent, but also 
in the fossil shells, in which latter a suspicion might possibly 
arise, of their fibrous structure being the consequence of their 
petrifaction ; but their fossil structure corresponding with their 
natural one, renders such a conjecture superfluous, although it 
is not to be doubted that the natural texture of animal bodies 
does frequently give the direction to the sparry matter which 
afterwards occupies those substances in the earth. 
