112 
THE PEARL OYSTER 
one good side and have to be chipped ofi the shell. They 
therefore command but a low price. 
The lustre or " water " of the pearl is due entirely to 
the action of light, and is dependent consequently on the 
Nacre being of a character to reflect light to advantage, and 
it is in this quality that the Nacre of the pearl oyster of the 
Gulf of Manaar excels all others, and it is for this reason 
that the pearls of our Avicula fueata are considered superior 
to all others in lustre. Probably the Nacre is laid on in 
thinner more translucent coats than in other pearls, and 
the lustre or so-called " water " may in some measure be 
due also to the clearness of the sea, for in fresh water pearl 
bearers, the presence of iron in the water imparts to the 
pearl an obvious red tinge. The iridescence of fish scales 
is referrible to the same cause, to wit the reflection of light 
from innumerable minute angles, and the cause being known, 
like effects have in a less degree of excellence been repro- 
duced in manufactures. 
Though Nacre is correctly called the mother-of-pearl, it 
is not by our pearl oyster Avicula fucata that the mother-of- 
pearl of commerce is yielded. Avicula fucata is not large 
enough, nor is the Nacre thick enough to produce the buttons 
and knife handles of commerce, and it is to the giant pearl 
oyster, Avicula margaritifera, that we have to look for that 
pirrpose, though in consequence of the name A. margaritifera 
having by many authorities been applied to our A. fucata, 
A. fucata is wrongly credited with producing mother-of- 
pearl. This error of name has been made and adopted on 
BO many able hands — not a few of them authorities entitled 
to marked deference — that I naturally hesitated long before 
presuming to conclude that not a little confusion has arisen 
from treating Avicula margaritifera and A. fucata as one 
and the same animal. The shell of the former is 7 uiches 
across, that of the latter 2>\ at matui'ity. 
