PEARLS AND IMITATION PEARLS 95 



rough, but the interior is such smooth mother-of- 

 pearl that it does not injure their delicate bodies. 



The shells of all mollusks are constructed of cal- 

 cium carbonate and other materials extracted from 

 the surrounding medium by the shell-fish. The mem- 

 branous covering of the mollusk, called the mantle, 

 takes the carbonate of lime and other mineral ele- 

 ments from the water and deposits them at the 

 different parts of its folds in the form of the shell. 

 Nature takes an infinite amount of care in arranging 

 the crystals of calcium carbonate in such a way as 

 to form mother-of-pearl. The calcium carbonate 

 crystals are deposited in regular overlapping layers, 

 the shell being constantly enlarged at the edge by the 

 exudations of the mantle, which deposits a protein 

 called conchiolin for the epidermis, and calcium car- 

 bonate for the prisms and inner layers of trans- 

 parent plates, until the shell has attained its growth. 



Placed under a microscope, the outer shell is seen 

 to be composed of a large number of overlapping 

 horizontal composite plates, which penetrate the shell 

 to the mother-of-pearl. The edge of a composite plate 

 appears as a number of prisms placed side by side 

 lengthwise across the plate edge, but showing dark 

 intersecting lines where the plates divide. On the 

 face of the plates these prisms appear as translucent 

 hexagons cemented together by darker organic 

 matter. The outer portion of the shell contains much 

 more organic matter and is of coarser texture than 

 the inner part of the shell. In certain species of shell- 

 fish the exterior of the shells is composed of nearly 

 pure conchiolin, a horn-like organic substance. In 



