346 ,
"
THE SEAS
with the Romans, they are mentioned in the literature of
most of theearly 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 shellsh and discovered in them round beads of
-
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 nest_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 scientic
_
investigation in the laSttwo 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 fermation to the action of lightning.
’
But it has been known for some hundreds of
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
ne 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
the soft “ mantle
”
which lies against it and
produces it, and there is some evidence that true pearls,