STRUCTURE OF SHELL. 
21 
and '0207 gram of organic matter, or 9G'96 per cent, of inorganic 
substances and .8 '04 per cent, of organic matter. 
Tlie calcareous portion of the shell 
has always an organic basis, which is 
first secreted and then gradually 
impregnated with the carbonate of 
lime, which latter substance is some- 
times deposited in layers of very 
distinct character. The organic basis, 
which is termed Conchyolin, main- 
tains the life of the shell, for on the 
death of the animal it soon disappears and the shell gradually becomes 
almost pure carbonate of lime, and very brittle ; there are, however, 
in some species traces of carbonate of magnesia, silicic acid, phosphate 
of lime and minute quantities of other substances. The specific gravity 
of shells generally is somewhat higher than that of Carrara marble, 
and many are perceptibly harder than, and will scratch, calc-spar. 
The inner layer of the shell, when duller than in nacreous shells, is 
termed porcellanous, but it is often iridescent or pearly, this charming 
appearance being caused by the peculiar disposition of the shelly matter 
in thin overlapping la 3 mrs with minutely corrugated edges, which 
diffract the light and thus causa the iiide.scent effect, the thinner and 
more ti’ansparent the overlapping plates, 
the more beautiful the lustre. This inner 
layer is secreted by the whole surface of the 
mantle and, according to Mr. Carrington, 
owes its exquisite smoothness in Hell.r 
jioiiKitid to a surface deposit of almost pure 
silica. The free glandidar margin secretes 
or forms the middle or prismatic layer, 
consisting of prisms of carbonate of lime 
arranged in a particular way. 
The outer surface, epiconch, or periostracnm, perhaps better known 
as the epidermis, is also a product of the collar of the mantle ; it is 
of a chitinons nature with a complex chemical composition said to be 
represented by the chemical formula CisHouNoOioi Rud varies 
remarkably in its thickness and general character, appearing to be 
analogous to the periosteum of the bones of the Vertebrates. It is 
Fig. 16. — Surface of Nacre 
or Pearl, highly magnified, 
allowing tlie iinnute undula- 
tions upon whici) the pearly 
lustre depends (after Tryon). 
Fig. lo.— Oblique section through the 
shell of L'nio (magnified), showing the 
three layers of which the shell is com- 
posed (after Semper). 
e. periostracum or epidermis, the ex- 
ternal layer; prismaticor middle layer; 
n. nacreous or inner layer. 
