12. Pearl Formation in the Indian Pearl Oyster. 
By JAMES HORNELL. 
recent results obtained by others, to stand by themselves as 
strictly conformable to the actual facts. 
Pearls in the widest sense may be defined as more or less 
rounded masses of shell substance made up of concentric 
layers laid down around a nucleas. The shell substance may 
e of any one of the four layers normally present in such 
shells as the pearl oyster or two or more of these may alternate 
in the layers. Some pearls consist wholly of periostracum ; 
these are brown and on account of the lack of lime in their com- 
sometimes found in the edible oyster. Hypostracal pearls are, 
in my experience, the most numerous of all in the local pearl 
Many of these become the pseudo-nuclei of nacreous see 
pearls, the real nuclei being of course the nuclei of the calco- 
spherules themselves. Not infrequently contiguous pearls of 
this nature fuse into a compound mass 0 irregular shape, one 
form of the baroque pearl, useful to the imaginative jeweller for 
