366 THE OCEAN. 
shells produce pearls of greater or less perfection; 
but what is known as the Pearl Oyster is the Avicula 
margaritifera of conchologists. The interior surface 
of the shell is covered with very thin. plates, or 
lamelle, which are furrowed with microscopically 
minute and close parallel grooves, and in this struc- 
ture lies the property of reflecting opaline tints; 
a property which has: been communicated to other 
substances by mechanically impressing the surface 
with similar grooves. In some diseased states of 
the animal, or when the shell has received a trifling 
injury, or some foreign body—a grain of sand, for 
example—has found its way within the mantle, the 
pearly secretion is poured out in great abundance 
around the part, and, layer being imposed upon 
layer, produces a pearl, either attached to the inner 
surface of the shell, or loose and held merely in the 
folds of the mantle. 
The most productive fishery is in the Persian 
Gulf, and the finest pearls are found there: ‘above 
90,0002. sterling are sometimes realized from this 
source in the course of two months. Those with 
which we are most acquainted, are carried on on 
the coasts of Coromandel and of Ceylon; the former 
being in the hands of the East India Company, the 
latter in those of the British Government. The 
Ceylon fishery has been well described by Captain 
Percival, the Count de Noe, and lately by Mr. Ben- 
‘nett. As the banks would soon be exhausted if 
fished every year, portions only are selected in turn, 
while the rest remains untouched to be recruited. 
In the month of November, the Government ap- 
