P THE MOLLUSCA 
mineral constituent is carbonate of lime, but from 1 to 2 per cent of 
phosphate of lime is also present, and traces (less than 4 per cent) 
of carbonate of magnesium. The greater part of the thickness of 
the shell is made up by the ostracum, which consists of two layers : 
(1) an external layer, frequently coloured, and formed by prisms of 
calcite ; (2) an internal layer consisting of arragonite, generally in 
the form of overlapping plates: it is the internal layer that forms, 
in various species, the nacre or mother-of-pearl. 
The growth of the shell is effected in two ways. Its extent is 
increased by the addition of new matter, secreted by the thickened 
edge of the mantle, to the outer or prismatic layer. Its thickness 
is increased by addition of new matter, secreted by the whole 
surface of the mantle, to the inner or nacreous layer. In addition 
ot 
Fic. 1. 
Young embryo of Purpura lapillus, from the left side, x 16. a, anus; f, foot; m, mouth ; 
ot, otocyst ; p.a.c, post-anal cilia ; re, embryonic kidney ; sh, shell; ve, velum; vi, vitellus. 
to the two layers of the ostracum already mentioned, there is a 
third layer, called the hypostracum, on the areas of attachment of 
the muscles which serve to fasten the animal to its shell. Finally, 
there is the periostracum or shell epidermis, forming an external 
covering to most shells: it is formed by the pallial fold at the 
edge of the mantle. In some Polyplacophora, Gastropoda, and 
Lamellibranchia, and in most of the Cephalopoda, the free edges of 
the mantle are reflected over the shell, so as to cover a greater or less 
part of its outer surface. In some species in which this special 
feature is developed to its greatest extent, the reflected mantle 
edges form a completely closed sac round the shell, so that the 
latter is internal and concealed. In rare cases the pedal integument 
may secrete a calcified “shell,” which may be adherent (Hipponyz) 
or free (Argonauta). 
The shell was for a long time the criterium of Molluscan 
classification. But the progress of anatomical study showed that 
