V] 



PEARLS 



55 



organic basis. The layers are arranged in a more or 

 less concentric manner, round a central nucleus, and 

 the four types of structure met with in the shell may 

 occur in the pearls. 



Fig. 5. Diagram showing the positions most frequently occupied by 

 muscle pearls in the Ceylon Pearl Oyster. (After Herdman.) 



These layers, it will be remembered, are the 

 periostracum (the most external, almost entirely 

 organic layer), the prismatic layer, the nacreous layer 

 (or mother-of-pearl substance), and the hypostracum 

 (the layer found where the large closing muscles are 



