IN GENERAL. 687 
In many, especially bivalve shells, a horny, brown-yellow 
outermost covering is found, which has been named epidermis or 
recently periostracum also (nm French Drap marin). Sometimes it 
is hairy or divided into scales, which however are more apparent at 
the margin of the shell, having been worn off from the parts pre- 
viously formed. This membrane has been regarded as a continua- 
tion of the cuticle of the mantle, by which this is connected with 
the margin of the shell. More correctly, perhaps, this covering may 
be explained from a confluence of the intercellular matter, a resi- 
duum of the formless homogeneous substance (the cytoblastema)}, in 
which the cells filled with lime were formed. Let it be supposod 
that at the outer margin of every layer this substance remains 
without cells, and consequently hardens like horn. If these edges 
should close upon each other, then a smooth epédermis will arise; if 
they should remain more distant from each other, then a scaly, floc- 
culent or hairy covering will be formed. 
The colours, presented by bivalve and univalve shells, are de- 
posited only in their outermost parts, the inner layers are white. 
This may be explained by the circumstance, that the colouring 
matter is secreted especially by the edge of the mantle. But the 
mantle grows with the animal, and thus each succeeding layer of 
the shell is coloured at its outer margin alone, whilst its remaining 
portion, secreted by the rest of the surface of the mantle, remains 
white. In this way a series of coloured edges arises, which, closing 
upon each other, form the outermost coloured surface of a shell. 
There are however some univalves (the genus Cyprea and some 
Olive) m which, when full grown, the colours are deposited not on 
the surface alone, but also in a deeper layer, whilst at the same 
time the superficial and the more deeply lying colours are different. 
These molluses are at first covered with a thin shell, of which the 
colours must be ascribed to the edge of the mantle. As the animal 
grows, lateral appendages of the mantle are developed, which throw 
themselves like wings over the shell, and secrete on their outer 
Reports of the British Association for 1844 and 1847. The chief particulars of these 
investigations may be also found in the Article Shell by the same writer in Topp’s 
Cyclopedia, Iv. 1849, p. 556, &c. 
1 Page 17, 18. 
