Shell of Fresh-water Mussel. 55 



themselves made up of polygonal masses of small size secreted by 

 the mantle exclusively of its margin. Very frequently, even in the 

 dry shelly it is possible to separate a thin white layer consisting 

 of a structureless membrane, laden with granular calcareous deposit 

 from the nacreous or ' mother of pearl •* internal layer of the shell, 

 which was formed by the successive super-impositions of such 

 membranes from within outwards. The free rim of the shell is 

 constituted by the coloured opaque epidermal layer, which forms a 

 flexible border of several lamellae, immediately internally to which, 

 in this, and in all still growing shells, on the inner surface of the 

 shell, a similarly dark-coloured but non-flexible strip intervenes 

 between the epidermal free border and the nacreous surface of the 

 shell. This portion of the shell which bounds the nacreous portion 

 like a fringing reef, is seen, when looked at with a simple lens, 

 to possess an appearance like that of shagreen, and this appearance 

 on examination under a higher power, is foiind to be due to its 

 being divided into minute polygonal spaces, over which the nacreous 

 layer has not yet extended. The inner of the two layers of which 

 this portion of the shell is made up, is known as the ' prismatic^ 

 layer, and is formed by the deposition of calcareous matter in the 

 interior of vertical prismatic cavities, which are themselves formed 

 by the successive super-addition and coadaptation of fenestrated la- 

 minae secreted by the margin of the mantle. There is less difference 

 between the thicknesses of the two inner layers of the shell in 

 the Anodon than in most other Lamellibranchiata ; the middle 

 or prismatic layer is, as the fact of its being secreted by the margin 

 of the mantle would have led us to anticipate, thickest, both 

 absolutely and relatively, in those parts of the shell in which 

 peripheral growth is carried on at the most rapid rate; and con- 

 sequently it is thicker at the free edges than along the dorsal 

 region, and in the posterior than in the anterior parts of the shell. 

 The relation held by the ' prismatic^ layer of the shell to the parts 

 where growth is going on most rapidly ; its existence there inde- 

 pendently of the ' nacreous^ layer ; and, thirdly, the fact that the 

 calcareous particles in its substance do not possess a crystalline 

 character, appear conclusive against the viewi which has been 



"I See H. Miiller quoted hy Hassling, 'Die Perlmuscheln und ihre Perlen,' p. 261, 

 Bronn and Keferstein, iii. 2, 913, and Rose, Berlin Akad. Abhand. 1858, p. 98. 



