BIVALVE MOLLUSCA. 369 
off their external surface plates of nacre are obtained more or less 
thick, according to the age of the mollusc. 
Nacres of three kinds are distinguishable in commerce: silver- 
lipped, bastard white, and bastard black. The first are sold in cases 
of 250 to 280 pounds; they are brought from the Indies, from China, 
and Peru. The ships of various nations import these shells as 
ballast. The second is delivered in casks of 250 pounds weight ; it 
is a yellowish white, and sometimes greenish ; sometimes red, blue, 
and green. 
Pearls form by far the most important product of the animal. 
When they are adherent to the Valves they are detached with pincers; 
but, as a'rule, they are found in the soft tissues of the animal. In 
this case the substance is boiled, and afterwards sifted, in order to 
obtain the most minute of the pearls ; for those of considerable size 
are sometimes overlooked in the first operation. Months after the 
mollusc is putrefied miserable Indians may be observed busying 
themselves with the corrupt mass, in search of small pearls which 
may have been overlooked by the workmen. 
The pearls adherent to the valve are more or less irregular in their 
shape ; they are sold by weight. Those found in the body of the 
animal, and isolated, are called virgin pearls, or paragons. They are 
globular, ovoid, or pyriform, and are sold by the individual pearl. In 
cleaning them, they are gathered together in a heap in a bag, and 
worked with powdered nacre, in order to render them perfectly pure 
in colour and round in shape, and give them a polish; finally, they are 
passed through a series of copper sieves, in order to sizethem. These 
sieves, to the number of twelve, are made so as to be inserted one 
within the other, each being pierced with holes, which determine the 
size of the pearl, and the commercial number which is to distinguish it. 
Thus, the sieve No. 20 is pierced with twenty holes, No. 50 with fifty 
holes, and so on up to No. 5,000, which is pierced with that number of 
holes. The pearls which are retained in Nos. 20 to 80, said to be mi//, 
are pearls of the first order. Those which pass and are retained be- 
tween Nos. 100 to 800 are vivadoe, or pearls of the second order; and 
those which pass through all the others and are retained in No. 1,000 
belong to the class /vo/, or seed pearls, and are of the third order. 
They are afterwards threaded ; the small and medium-sized pearls 
on white or blue silk, arranged in rows, and tied with ribbon into a 
top-knot of blue or red silk, in which condition they are exposed for 
sale in rows, assorted according to their colours and quality. The 
small or seed pearls are sold by measure or weight. 
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