596 PRISMATIC SHELL-SUBSTANCE. 



when a sufficiently high magnifying power is used, are seen to be 

 minute grooves, apparently resulting from a thickening of the in- 

 termediate 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 ten- 

 dency to split into thin lamina? 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 coalesced, but for some accidental cause which pre- 

 served 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 extremities, would appear also from 

 the fact that where the 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 colourless. 



459. This Prismatic arrangement of the Carbonate of Lime in 

 the Shells of Pinna and its allies, has been long familiar to Con- 

 chologists, and regarded by them as the result of Crystallization. 

 When it was first more minutely investigated by Mr. Bowerbank* 

 and the Author, +" and was shown to be connected with a similar 

 arrangement in the Membranous residuum left after the decalcifica- 

 tion of the Shell-substance by acid, Microscopists gene rally + 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, however, has 

 led to an important modification of this interpretation ; the Author 

 being now disposed to agree with Prof. Huxley§ 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,'|| 

 being here thrown out at the same time with the calcifying mate- 

 rial, 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 



* ' On the Structure of the Shells of Molluscous and Conchiferous Ani- 

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



t 'On the Microscopic Structure of Shells,' in "Reports of British 

 Association" for 1844 and 1847. 



J .See .Mr. Qnekett's " Histological Catalogue of the College of Surgeons' 

 Museum," and lus "Lectures on Histology," Vol. ii. 



§ See his article 'Tegumentary Organs,' in " Cyclopaedia of Anatomy 

 and Physiology," Supplementary Volume, pp. 480-492. 



|| The peHostraewm is the yellowish-brown membrane covering the 

 surface of many shells, which is often (but erroneously] termed their 

 epidermis. 



