22 
STRUCTURE OF SHELL. 
very persistent and almost indestructible and has been observed still 
adherent to tertiary fossils ; its chief olhce -would appear to be to 
protect the calcareous portion of the shell from the disintegrating 
influence of external conditions, but on the death of the animal, if the 
shell is left exposed to the weather, this epidermis is very quickly 
deciduous, though practically persistent during the life of the animal. 
This protective covering is strongly developed and remarkably 
varied in freshwater shells, and in land species habitually fre- 
quenting moist and shaded situations, it is also sometimes the seat 
of the colouring matter with which shells are often ornamented, 
but usually the pigment is resident in the calcareous portion of 
the shell, the epidermis or i)eriostracum being generally of a trans- 
parent or semi-transparent horny tint, through which the colouring 
is seen more or less vividly. In some of the Philippine Bid'nni the 
epiconch or epidermis is double, that is formed of two distinct layers, 
and this peculiarity has been showii by i\Ir. 'I’ye to be shared by at 
least one British species, the IlcJi.r (n-h/isfonim ; this duplication of 
the periostracum is best seen in the variety .// i'Nywo/.s’, as the less 
persistent external film is in that form darker in colour than the 
inner layer, it however very quickly exfoliates, and is therefore 
freciuently found only in more or less isolated and irregular i)atches, 
chiefly upon the body whorl. 
The varied sculpture and projecting processes which ornament 
many species, and are sometimes especially indicative of maturity, 
or of periodic cessation of growth, have often their internal support 
and receive their character and form from the calcareous portion of 
the shell, but the hairy or bristly appendages which distinguish many 
species would appear to be solely epidermic in origin and character. 
Giimbel has demonstrated by experiment the enduring character of 
the outer or epidermic layer, and shown that carbonic acid gas dis- 
solved the compact portions of shells more (juickly than those less 
closely aggregated, agreeing thus with the observations of geologists, 
which tend to show that the outer layer of shells is better preserved 
than the nacreous, and this than the fibrillar layer. 
If the carbonate of lime in a shell be removed by acid, the organic 
residuum may still retain the shape of the shell, forming a sort of 
membranous framework, as the chitinous epiconch is scarcely affected 
by ordinary acids, though dissolved by caustic alkali. Mr. Crowther 
