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 Helices, 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 pi'oof 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 epidermis, 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 Periostraca, 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 us6 

 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. 210. 



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