194 SHELLS AND MOLLUSCOUS ANIMALS. 



pearl, made of gum arable, sealing-wax, or shell- 

 lac, have been made to assume the shiny appear- 

 ance of that substance. Pearls are secreted, not 

 only by the oyster and mussel, but also by 

 several bivalve mollusks ; and they are of the 

 same substance as the nacre, consisting of animal 

 matter and carbonate of lime, arranged in con- 

 centric layers around a nucleus. They appear to 

 be originated by some local interruption to the 

 comfort of the animal. Thus small grains of 

 sand by some accident get in between the valves 

 of the shell, and become encrusted with layers of 

 nacre, slowly increasing and assuming the round 

 shape of these beautiful gems ; or the annelidous 

 worm pierces a hole in the shell, and the oyster 

 forms a little plug to stop up farther intrusion. 

 Linnseus said that he could make pearls at plea- 

 sure, by perforating the shell with a pointed wire, 

 and introducing grains of sand. 



" Pearls." says Mr. Gray, " are usually of the 

 colour of the part of the shell to which they are 

 attached. I have observed them white, rose- 

 coloured, purple, and black, and they are said to 

 be sometimes of a green colour. They have also 

 been found of two colours ; that is, white with a 

 dark nucleus ; which is occasioned by their being 

 first formed on the dark margin of the shell, 

 before it is covered with the white and pearly coat 

 of the disk, which, when it becomes extended over 

 them and the margin, gives them that appearance." 



The frequent mention of pearls by the Scripture 

 writers, shows how highly they have ever been 

 esteemed among Oriental nations. Indeed, in the 

 East, they seem to have been prized even more 

 than the brilliant diamond. " In the West," says 



