STRUCT UKE OF SHELLS 



9 2I 



additions at its base, the lines of junction of which correspond with 



the transverse striation ; and this view corresponds well with the 



fact that the shell -membrane not unfrequently shows a tendency to 



split into thin laminae along the lines of striation, whilst we occa- 



sionally meet with an excessively thin natural lamina lying between 



the thicker prismatic layers, with 



one of which it would have 



probably coalesced but for some 



accidental cause which preserved 



its distinctness. That the prisms 



are not formed in their entire 



length at once, but that they are 



progressively lengthened and 



consolidated at their lower ex- 



tremities, would appear also 



from the fact that where the 



FIG. 695. Section of the shell of Pinna 

 in the direction of its prisms. 



shell presents a deep colour (as 



in Pinna nigrina) this colour 



is usually disposed in distinct 



strata, the outer portion of each layer being the part most deeply 



tinged, whilst the inner extremities of the prisms are almost colour- 



less. 



This * prismatic ' arrangement" of the carbonate of lime in the 

 shells of Pinna and its allies has "been long familiar to concholo- 

 gists, and regarded by them as the result of crystallisation. When 



FIG. 696. Oblique section of prismatic shell-substance. 



it was first more minutely investigated by Mr. Bowerbaiik 1 and the 

 Author, 2 and was shown to be connected with a similar arrangement 

 in the membranous residuum left after the decalcification of the shell - 

 substance by acid, microscopists generally 3 agreed to regard it as a 

 ' calcified epidermis,' the long prismatic cells being supposed to be 

 formed by the coalescence of the epidermic cells in piles, and giving 



1 ' On the Structure of the Shells of Molluscous and Conchiferous Animals,' in 

 Trans. Microsc. Soc. ser. i. vol. i. 1844, p. 123. 



2 ' On the Microscopic Structure of Shells ' in Reports of British Association for 

 1844 and 1847. 



See Mr. Quekett's Histological Catalogue of the College of Surgeons' Museum 

 and his Lectures on Histology, vol. ii. 



