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 Wa“otis iris, for instance, has 
a nacre of an emerald-greenish blue, changing colour with reflections 
of a purple violet. Zurbo argyrostomus (Linnzus) presents a mouth 
of bright silvery hue, while Zurbo chrysostomus appears in all the 
glory of gold; but the Pxtadine yields the purest white nacre, as 
well as the most uniform, and especially the thickest. ‘This product 
Outside of the shell. Inside o: the shell. 
Fig. 163. —Meleagrina margaritifera (Linnzeus). 
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 d¢/ea- 
