CHAPTER XVII. 



Tn i PARTICULAR KIND OF "CRIPPLED'' SHELL THAT 

 YIELDS THE PEARL. 



PEARLS are found in various molluscan shells, but 

 the ones of the greatest beauty and value are 

 found in shells which have irridescent interior 

 layers. Of the shells which have the pearly interior the 

 most important are the Aviculadae, Mytilidae, and the 

 Unionidae. 



The first two groups are marine shells, while the last is 

 the fresh-water group which are found in the rivers of 

 many lands and are most abundant in our country. 



In the marine mollusks, or oysters, the two valves are 

 not of the same size and shape. The normal valves of 

 fresh-water mussels, however, are of the same size and 

 shape and very seldom contain pearls of value, although 

 in rare instances very fine perfect pearls have been found 

 in them. 



The fine pearls are usually found in what are known as 

 "crippled" shells. There are no two "cripples" that ^re 

 precisely the same in size and shape, yet there are two 

 distinct classes of them, which may be termed as the 

 worthless "cripples" and the valuable "cripples." 



The "crippled" shells have an abnormal appearance. 

 Such mussels have a long convex elevation on the ex- 



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