BIVALVE MOLLUSC A. 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 size them. 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. r,ooo, which is pierced with that number ot 

 holes. The pearls which are retained in Nos. 20 to 80, said to be mill, 

 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 tool, 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 

 ".op-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 



