STKUCTUEE OF SHELLS 



921 



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 lamina 1 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 



shell presents a deep colour (as 



D f . . . L . FIG. 695. Section of the shell of Pinna 



in / tuna mgmna) this colour in the direction of its prisms. 



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 coiicholo- 

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



FIG. 69(5. Oblique section of prismatic shell-substance. 



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

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

 in the membranous residuum left after the de-calcification 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 



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

 Trans. Mir ><>.. i'oc. ser. i. vol. i. 1344, p. 123. 



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

 1*41 and 1847. 



See Mr. Quekett's Histologicul Catalogue of the College of Surgeons' 

 iind his Lrrtiin-s on Hivtoliit///, vol. ii. 



