300 DR. H. LYSTER JAMESON OX 



(iii.) the Naci'e 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 crystalline carbonate of lime deposited therein. Romer's 

 careful 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 forixi being determined by that of the spaces in 

 which it is deposited. 



The ratio of conchyolin to calcareous salts differs in different 

 parts of the shell. Thus Romer (32) has found in Margaritana, 

 the fresh-water pearl-mussel, that the organic substance con- 

 stitutes 1'47 per cent, by weight of the prismiatic 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 of 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 sevei'al 

 constituents of the shell. 



(i.) The Periostrc{cum. 



The origin of the Periostracum can best be understood if we 

 consider first those forms which live in fresh or estuarine water, 

 or are otherwise subjected to conditions which render necessary 

 a thick cuticle-like layer to defend them from the erosive action 

 of organic acids derived from decomposing animal and vegetable 

 matter (e. g. the Unionidye and Mytilus). 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 Midler (29), Stempell (44), List (27 6), 

 etc. 



In these cases the periostracum is composed of two constituents. 

 The outermost layer is probably formed as a true cuticle directly 

 by transformation 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 Yillepoix (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 pai't of the shell-secreting apparatus. This outer 



