V] 



PEARLS 



57 



like the h3'postracal layer of the shell. Radiating 

 striae pass outwards, and in addition a delicate 

 concentric lamination can be observed. 



Fig. 6. Section of a pearl composed of prismatic substance with 

 nucleus of periostracum-like substance. (From Eubbel.) 



Pearls of larger size, consisting almost entirely of 

 prismatic-like shell substance are more common, but 

 it must be remembered that usually the periostracal 



