300 DR. H. LYSTER JAMESON ON 
(ili.) the Naere or Mother-of-Pearl, forming the lining and 
the bulk of the shell ; 
(iv.) the Hypostracum, the substance to which the muscles 
are attached by a specialised epithelium ; 
(v.) the Hinge Ligament. 
The mass of the shell is further divisible chemically and 
microscopically into an albuminoid substance called “ conchyolin ” 
and erystalline carbonate of lime deposited therein. Roémer’s 
‘areful observations (32) have shown beyond a doubt that there is 
a sharp separation between these two substances : the conchyolin 
forming an alveolar framework, in the chambers of which the 
salts are deposited ; the structure of the calcium carbonate being 
crystalline, its form being determined by that of the spaces in 
which it is deposited. 
The ratio of conchyolin to caleareous salts differs in different 
parts of the shell. Thus Rémer (32) has found in Margaritana, 
the fresh-water pearl-mussel, that the organic substance con- 
stitutes 1°47 per cent. by weight of the prismatic substance, but 
only *64 per cent. of the nacre. This is most interesting as 
giving support to the theory of the present writer, enunciated 
below, that the different structures of the different forms of 
shell-building substances, normal and pathological, are in part 
a function ae the proportions in which these two constituents are 
secreted by the tissues of the mollusc. 
To turn now to the details of the structure of the several 
constituents of the shell. 
(1.) The Periostracum. 
The origin of the Periostracum can best be understood if we 
consider ae those forms which live in fresh or estuarine water, 
or are otherwise subjected to conditions which render nece ssary 
. thick cuticle-like layer to defend them from the erosive action 
of organic acids derived from decomposing animal and vegetable 
mmatter (e.g. the Unionidee and J/ytilus). The periostracum 
in such cases has been described fully by several authors, 
e.g. Biedermann (1), Moynier de Villepoix (28), Tullberg (47), 
Ehrenbaum (9), Felix Miiller (29), Stempell (44), List “(27 b), 
ete. 
In these cases the periostracum is composed of two constituents. 
The outermost layer is probably formed as a true cuticle directly 
by transfor mation or cuticularisation of the outer surfaces of the 
cells of a specialised epithelium in the inner (axial) face of a deep 
groove which runs along the mantle-margin, and which has been 
called by Moynier de V ‘illepoix (28, p. 18) the “ fente marginale.” 
This marginal groove divides the mantle-margin into an inner 
and an outer lobe, the former being pigmented and sensory, the 
latter being a part of the shell- secreting apparatus. This outer 
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