MOLLUSCOUS ANIMALS GENERALLY. 173 



they can be readily detached one from another; and each one may be 

 observed to be marked by the like striations, which, when a sufficiently 

 high magnifying power is used, are seen to be minute grooves, apparently 

 resulting from a thickening of the intermediate wall in those situations. 

 These appearances seem best accounted-for, by supposing that each is 

 lengthened by successive 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 

 occasionally meet with an excessively thin natural lamina lying between the 

 thicker prismatic layers, with one of which it would have probably coa- 

 lesced, 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 extremi- 

 ties, would appear aL=o from the fact that where the shell presents a deep 

 color (as in Pinna nigrina), this color 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 colorless. 



564. This ' prismatic' arrangement of the carbonate of lime in the 

 shells of Pinna and its allies, has been long familiar to Conchologists, 

 and regarded by them as the result of crystallization. When it was first 

 more minutely investigated by Mr. Bowerbank 1 and the Author, 3 and 

 was shown to be connected with a similar arrangement in the membran- 

 ous 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 their shape to the deposit of 

 carbonate of lime formed within them. The progress of inquiry, how- 

 ever, has led to an important modification of this interpretation; the 

 Author being now disposed to agree with Prof. Huxley 4 in the belief 

 that the entire thickness of the shell is formed as an excretion from the 

 surface of the epidermis^ and that the horny layer which in ordinary 

 shells forms their external envelope or ' periostracum,' 5 being here 

 thrown out at the same time with the calcifying material, is converted 

 into the likeness of a cellular membrane by the pressure of the prisms 

 that are formed by crystallization at regular distances in the midst of it. 

 The peculiar conditions under which calcareous concretions form 

 themselves in an organic matrix, have been carefully studied by Mr. 

 Eainey and Dr. W. M. Ord; of whose researches some account will be 

 given hereafter ( 711). 



565. The internal layer of the shells of the MargaritacecB and some 

 other families has a ' nacreous' or iridescent lustre, which, depends (as 

 Sir D. Brewster has shown 8 ) upon the striation of its surface with a 



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

 " Transact, of Microsc. Society," 1st Ser. (1844), Vol. i., p. 123. 



2 ' On the Microscopic Structure of Shells, 1 in " Reports of British Association " 

 for 1844 and 1847. 



3 See Mr. Quekett's " Histological Catalogue of the College of Surgeons' Mu- 

 seum," and his " Lectures on Histology," Vol. ii. 



4 See his article ' Tegumentary Organs,' in "Cyclopaedia of Anatomy and Phy- 

 siology," Supplementary Volume, pp. 489-492. 



5 The Periostracum is the yellowish-brown membrane covering the surface of 

 many shells, which is often (but erroneously) termed their epidermis. 



6 "Philosophical Transactions," 1814, p. 397. The late Mr. Barton (of the 

 Mint) succeeded in producing an artificial iridescence on metallic buttons, by 



