MOLLUSCA: THEIR SHELLS. 



43 



of its surfaces has a sort of facetted, or honeycombed 

 appearance ; and the broken edges, which even to the 

 naked eye appear fibrous, are seen to resemble a number 

 of basaltic columns. " The shell is thus seen to be com- 

 posed of a vast number of prisms, having a tolerably 

 uniform size, and usually presenting an approach to the 

 hexagonal shape. These are arranged perpendicularly, or 

 nearly so, to the surface of the lamina of the shell ; so 

 that its thickness is formed by their length, and its two 

 surfaces by their extremities."* 



SECTION OF NACRE FROM PEARL OYSTI 



The inner layer of such shells is remarkable for pos- 

 sessing in different degrees the property of reflecting rain- 

 bow-like colours, often with great delicacy and splendour ; 

 and this is termed nacre, or familiarly " mother-of-pearl." 

 This iridescent lustre depends, as Sir David Brewster has 

 shown,! upon a multitude of grooves or fine lines, which 

 run in a very waved pattern, but nearly parallel to each 

 other, across the surface of the nacre. "As these lines 

 are not obliterated by any amount of polishing, it is 



* Carpenter, " The Microscope," 590. f " K^ Trans.," 1814. 



