96 THE WEALTH OF THE SEA 



some mollusks this epidermal covering may be 

 stripped off easily, while in others it cannot be sepa- 

 rated from the plates of calcium carbonate prisms. 

 In the interior of the shell the plates of the calcium 

 carbonate prisms are much thinner and are attached 

 together so firmly that they cannot easily be sepa- 

 rated. The inner series of these plates as they near 

 the nacreous lining become harder and more compact, 

 so that the exact place where they join the nacreous 

 layer is difficult to determine. The nacreous lining 

 of the shells has the same general composition as the 

 coarser outer part of the shell except that it contains 

 less organic matter. It is harder, finer, and more 

 compact than the outer shell, and is constructed of 

 nacreous plates which are composed of very thin 

 waves of calcium carbonate set in a small amount 

 of animal tissue. These waves are very numerous and 

 produce a series of fine irregular lines which tend 

 generally to be straight but often curve as do the 

 edges of the waves of water when they run on the 

 sands of the sea-shore. The overlapping of these 

 thin transparent waves, and the minute undulations 

 of the layer edges reflecting through the transparent 

 plates, produce the soft luster known as pearly. The 

 minute prisms of calcium carbonate also aid in pro- 

 ducing the chromatic or iridescent effect by splitting 

 the white light into the several colors of which it 

 is composed. 



Origin and Nature of Pearls 



The pearl and the nacre of the shell in which it 

 grows are of essentially the same composition and 



