Forms, Composition, and Structure of Shells. = 347 
of tropical seas and of tropical climes! How sober and sub- 
dued are those which inhabit our northern shores! The ter- 
restrial Hélices, being most exposed to the operation of light, 
vary most in their colours ; while those shells which are con- 
cealed within the bodies of their snails are always white, as are 
also those which live in holes whence they never issue. An- 
other striking proof of the blanching effect of darkness is fur- 
nished by some bivalve shells permanently affixed by their 
lower valve, which is constantly white, while the upper one 
may possibly be variegated with bright colours. The spon- 
dyli, and a number of pectens, afford examples of this con- 
trast between their valves. Olivi has further remarked, that 
the shells which are enveloped in sponges, or other foreign 
bodies, or which burrow in sand, or even which live in con- 
stantly shaded places, are much paler than those which crawl 
about unprotected from the light; and even the exposed 
parts of the same shell are more highly coloured than the 
parts which are shaded. 
There is only one other part which remains to be noticed. 
in this letter. ‘This is the epzdermis, or what the French call 
the drap marine. It is a dry horn-coloured membrane, of 
greater or less thickness, generally smooth, but sometimes 
rough, or even shaggy, which covers the external surface of 
most shells; though many, particularly those of the convolute 
kind, do not possess it. Some difference of opinion exists as 
to its nature. Blainville supposes that it is the true epidermis 
or scarf-skin of the animal, raised from its position by the 
deposition of the shell underneath it; and this opinion, we 
believe, is generally entertained as correct by the French na- 
turalists. It seems inconsistent, however, with the doctrine 
of the formation of shells explained above, and cannot, there- 
fore, be adopted by us. Mr. Gray, who calls this membrane 
the Peridstraca, it appears to me, has given its true theory. 
He says, ‘* From attentive examination, I am inclined to be- 
lieve that the periostraca is formed by the outer edges of the 
plates of animal matter (which has no cretaceous matter de- 
posited in it, or only a very small quantity) being soldered 
together, and forming a kind of external coat.” * It is, in fact, 
a dried sheet of coagulated albumen effused at the same time, 
or, perhaps, even prior to the first layer; and may be of use 
to protect the subjacent and more cretaceous layers from the 
action of the air or water during their consolidation. 
To conclude. When a shell has attained its full growth, 
the changes which the animal further effects are almost limited 
_ * See his very interesting Conchological Observations, in the first volume 
of the Zoological Journal, p: 216, : 
AA Z 
