256 OF BIVALVED SHELL-FISH. 



a chimney, through which the animal breathes, or imbibes the fea-water. 

 At low water, thefe holes are eafily diflinguilhed by the filhermen, 

 whofe method of enticing the razor up from the depth of its retreat is 

 by fprinkling a little fea-falt on the hole. This melting, no fooner 

 reaches the razor below than it rifes inftantly ftraight upwards, and fhews 

 about half its length above the furface. This is inftantaneous ; and if 

 the fifher does not feize the opportunity, the razor buries itfelf with 

 great eafe to its former depth. There it continues fecure ; no fait cart 

 allure it a fecond time i but it remains unmolefted, unlefs the fifher 

 will be at the trouble of digging it out fometimes two feet below the 

 furface. 



In this clafs of fhell-fifh Pearls are found ufually generated, in the 

 internal parts of fhclls that are of a Ihining filvery colour, as well in the 

 muffel or the fcallop as in the oyfter, and in all bivalved fhells whofe in- 

 fidesrefemble mother-of-pearl. The oyfter that breeds a large pearl always 

 breeds a large fhelJ, and thefhell itfelf indents to receive its impreffion. 

 The pearl is probably accidentally produced by the fame matter as forms 

 the fhell. It is foft at firft, but quickly hardens ; and by fucceflive 

 coats, layer over layer, acquires its dimcnfions. Sometimes a fmall 

 fpeck is feen in the middle, on which the coats were originally formed. 

 The pearl oyfter has a large ftrong whitifti ftiell, wrinkled and rough 

 without, within fmooth and filver-coloured. From thefe mother-of- 

 pearl is taken, which is the internal coats of the (hell. The pearl is 

 chiefly prized J being found but in few, generally adhering, fometimes 

 making a print in the body of the fhell, fometimes at large within the 

 fubftance of the fifh. 



There are many pearl fifheries in America and Afia ; but thofe of 

 America are greatly difcontinued. The moft famous of the Aliatic 

 fifheries is in the Perfian Gulph, and the moft valuable pearls are brought 

 from thence. Their value increafes in proportion to their fize, their figure, 

 and colour ; fome are white, others yellowifh, others a lead colour, and 

 fome black as jet. What occafions thefe different tindtures is unknown. 

 All pearls in time become yellow ; they may be confidered as an animal 

 fubftance, converted into aftony hardnefs, and like ivory taking a tindurc 

 from the air. They alfo decay if kept in damp or vaulted places, and 

 moulder into a fubftance fcarce harder than chalk. 



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