364 THE OCEAN WORLD. 



Nacre is the hard and brilliant substance with which the valves of 

 certain shells are lined in the interior. This substance is white, 

 silky, slightly azure, and more or less iridescent. Most of the bivalves 

 are supplied with nacre ; some of them even yield it of a blue, or 

 blue and violet colour. The iridescent Haliotis iris, for instance, has 

 a nacre of an emerald-greenish blue, changing colour with reflections 

 of a purple violet. Turbo argyrostomus (Linnaeus) presents a mouth 

 of bright silvery hue, while Turbo chrysostomus appears in all the 

 glory of gold ; but the Pintadine yields the purest white nacre, as 

 well as the most uniform, and especially the thickest. This product 



Outside of the shell. Inside of the shell. 



Fig. 163. Meleagrina margaritifera (Linnaeus). 



owes its brilliant and delicate appearance to the play of light on 

 it in its highly-polished state. For practical purposes the nacre 

 is separated from the shell with an instrument ; sometimes all the 

 exterior part of the shell being dissolved away from the precious 

 substance, leaving only the naked bed of nacre. 



The pearl oyster (Meleagrina margaritifera\ is the most interesting 

 of all the nacre-bearing shells ; the exterior as well as the interior of 

 the shell is represented in Fig. 163. The interior of the shell affords 

 the most exquisite pearls ; the Esterhazy collection of jewels contains 

 many magnificent specimens. This shell is nearly round, and 

 greenish in colour on the outside ; it furnishes at once the finest 

 pearls, under favourable circumstances, and the nacre so useful in 

 many industrial arts. Fine pearls and nacre have, in short, the same 

 origin. The nacre invests the whole interior of the shell of Melea- 



