110 
THE PEARL OYSTER 
obtaining and allowing also a margin for our ignorance as 
to the exact limits within which it obtains, it seems not wise 
to rely much on any rule of age, but rather to lift a sample 
of oysters every six months and to examine the size of the 
pearls they yield. 
There is yet again another reason why it is more prudent 
to be guided by actual investigation made every season than 
by any general rtde of age. This lies in the belief that as 
its reproducing powers weaken with age the oyster absorbs 
and re-utilizes, for other purposes, the very pearl which it had 
thrown out in the vigor of its youth and middle age. This 
subject will be again referred to when remarking on the 
formation of pearls. 
Signs of Age. 
The practical test of the oyster having reached an age at 
which it is ready to be fished is, firstly, its having attained 
the full size of 3| inches rectangularly from hinge line to 
contour. Then the divers feel the edge of the shell, and, 
if it is pliable and will break ofE in the fingers, the shell is 
still growing and the oyster is too young. Having reached 
maturity, the signs of increasing age are increasing thickness 
of shells, and lastly, a V-shaped gape at the hinge. "WTien 
this is deep and wide the oyster is near its term of life and 
the fishery cannot be delayed. Gaping at the umbones is a 
mark of age in all bivalves. But these are only approximate 
guides ; and where it appears from them that the oysters are 
mature the time has arrived when the growth of the pearls 
should be periodically tested by lifting and examining a 
sample of oysters every six months. 
The Formation of Pearls. 
Like other molluscs the pearl oyster coats the interior of 
its shell with a smooth substance called Nacre. This it does 
for the purpose of protecting itself fi'om friction "«-ith uneven- 
