174 



THE ANIMALS AND MAN 



The extent of the acreage of the American oyster-beds is 

 larger than that of any other country. "The Baltimore 

 oyster-beds on the Chesapeake River and its tributaries 

 cover 3,000 acres, and produce an annual crop of 25,000,000 

 bushels." 



The "pearl-oyster" is not a true oyster, that is, not a 

 member of the family to which the edible oysters belong, 

 but it is a member of the same class, that is, it is a bivalve 



Fig. 85. Pholas sp., a mollusc burrowing in sandstone. (Photograph 

 by C. H. Snow; permission of the American Society of Civil Engineers.) 



mollusc. Pearls are obtained from a number of different 

 "pearl-oysters," but the finest pearls and mother-of-pearl 

 come from the tropical species Meleagrina margaritifera, 

 This pearl-oyster has an extensive distribution, being 

 found in Madagascar, the Persian Gulf, Ceylon, Austraha, 

 Philippine Islands, South Sea Islands, Panama, West 

 Indies, etc. Mother-of-pearl is simply the inner lining 

 of the shell, which is composed of numerous thin layers of 

 carbonate of lime so arranged that the edges of the suc- 

 cessive layers produce many fine strias very close together. 

 The beautiful iridescence of this inner shell-lining is caused . 

 by the complicated diffraction and reflection (interference 

 effects) of the light by the fine striae and the translucent 



