MOLLUSCA. 47 



have been so elaborately studied by Dr. Carpenter. In 

 general, these consist of two very distinct layers, well 

 seen in the valve of the Pearl Oyster, and its allies. 

 The Pinna, or Wing-shell, the largest of our native 

 bivalves, affords us a good example, especially of the 

 external layer, since here this layer projects beyond the 

 inner one, in thin transparent edges, which give us 

 an opportunity of examining their structure, without 

 any artificial preparation. This fragment, taken from 

 the edge of one of those leafy expansions, we will 

 examine with a low magnifying power. Each of its 

 surfaces has a sort of faceted, or honeycombed ap- 

 pearance, and the broken edges, which even to the 

 naked eye appear fibrous, are seen to resemble a num- 

 ber of basaltic columns. " The shell is thus seen to 

 be composed of a vast number of prisms, having a toler- 

 ably uniform size, and usually presenting an ap- 

 proach 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 extremi- 

 ties."* 



The inner layer of such shells is remarkable for pos- 

 sessing in different degrees the property of reflecting 

 rainbow-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,f upon a multitude of 

 grooves, or fine lines, which run in a very waved pat- 

 tern, but nearly parallel to each other, across the sur- 

 face of the nacre. " As these lines are not obliterated 



* Carpenter. The Microscope, p. 690. f Phil. Trans. 1814. 



