Molluscs 137 



much difficulty. This prince of com- 

 merce dredged his pearl oyster beds and 

 deliberately placed glass beads in all his 

 young healthy specimens, which were 

 replanted on special beds free from star- 

 fish, crabs and other marauders. Three 

 years sufficed to cover the bead with a 

 rich layer of nacre and, as a result, 

 pearls purchased to-day command a 

 much lower price than they did twenty 

 years ago. 



The tropical ' Thorny Oyster " a 

 '' bivalvular hedgehog " ranks amongst 

 the gems of the ocean. Unlike the pearl 

 oyster its beauties are all exterior. The 

 upper valve is covered with spines of 

 delicate sculpture, whilst the whole mol- 

 lusc is aflame with wonderful orange, 

 gold and scarlet tints that blend perfectly 

 with the various coral animals amongst 

 which it lives. 



The Mangrove Oysters attach them- 

 selves to branches that are submerged 



