56 PEARLS [CH. 



attached to the shell). Some pearls consist entirely 

 of concentric laminae of a substance like the peri- 

 ostracum. These pearls are not common but are 

 sometimes to be found near the very edge of the 

 mantle. Naturally, they are somewhat brown in 

 colour and lack entirely the characters which are 

 associated with pearls in the popular mind. 



Pearls consisting entirely of hypostracum-like 

 substance are to be found, often in large numbers, in 

 the muscles. In fact, many if not most of the muscle 

 pearls are composed of hypostracum. 



Now, where the muscle pearls are abundant, large 

 numbers of calcareous concretions or calcospherules 

 are scattered in the tissues. Herdman and Hornell 

 suggested that the muscle pearls were formed around 

 these microscopic calcospherules. It is extremely 

 probable, as suggested by Jameson, that these calco- 

 spherules are in reality small pearls — probably of 

 a hypostracum-like substance. Round them may 

 be deposited more layers of hypostracum or nacre. 

 There is no evidence in any case to support the 

 view of Mr Southwell that the calcospherules are 

 calcareous depositions from the blood. 



The pearls composed of hypostracum may possess 

 either an empty central cavity, a cavity containing 

 granules, or a small central nucleus of yellow^ peri- 

 ostracal-like substance. The calcium carbonate is 

 laid down round the centre in a manner strikingly 



