346 THE SEAS 



with the Romans, they are mentioned in the literature of 

 most of the early civilizations, so that to seek the beginning 

 of the use of pearls we should probably have to go back 

 before the birth of history to the days of primitive men who 

 lived on shellfish and discovered in them round beads of 

 great lustre and beauty. 



Many of the bivalve molluscs form pearls but in only a 

 very few are they of the necessary size and quality. The 

 principal source of the finest pearls is the pearl oyster which 

 is a very different animal from the edible oysters, all of 

 which belong to the genus Ostrea, whereas the Ceylon 

 pearl oyster (Plate 121) and its relatives in Australia, the 

 Malay Archipelago, the East Coast of Africa, the Persian 

 Gulf and elsewhere, all belong to the genus Margaritifera 

 and are more closely allied to our common mussel. 

 Valuable pearls are also found in fresh-water mussels in 

 this country and throughout Europe, also in America and 

 Asia, but they do not concern us here. 



The formation of pearls has always given rise to 

 speculation and has been the subject of much scientific 

 investigation in the last two or three centuries. It was the 

 belief of the ancient Hindoos that dew drops which fell 

 within when the oyster opened its shell were later converted 

 into pearls by the rays of the sun, while another early 

 theory attributed their formation to the action of lightning. 

 But it has been known for some hundreds of years now that 

 pearls are really due to some abnormal stimulation of the 

 tissues of the oyster, though the cause of this is still a matter 

 for discussion. Blister pearls, which are fastened by a 

 fine neck to the inside of the shell, and are often sawn 

 off and used in cheap jewellery, are probably produced by 

 sand grains which have worked their way in between the 

 shell and the soft " mantle " which lies against it and 

 produces it, and there is some evidence that true pearls, 



