THE AI1AL0XFS OF CALIFORNIA 



543 



pearly layer of the shell, formed by the secreting cells of the mantle in 

 defense of the invading, boring mollusk, Pholadidea parva. They occur 

 mostly in the red abalone, with only one blister-pearl in about a thou- 

 sand shells of the green or black species. A crab, which infests the 

 abalone at certain seasons, may be the cause of such formations, and one 

 exhibited the complete outline of such a crab. Frequently the blister- 

 pearls are formed over sea-urchin spines, chiton or razor-clam shells, 



Blistek-peakl Formed over a Diseased Visceral Hump. 



pebbles and other foreign bodies retained beneath the mantle. Some- 

 limes a diseased visceral hump is cut off and covered by nacre, making 

 a huge blister-pearl. 



The free pearls have the color of the inside layer of the shell, vary- 

 ing from white, to green, or pink, according to the species. They sell 

 from fifty cents, for the smaller ones, to one hundred and twenty-five 

 dollars for one of twenty-five grains. Occasional pearls are so large 

 and of such fine quality as to sell for five hundred, or even one thou- 

 sand dollars. The free pearls are frequently found within the stomach. 

 During the year 1912, over eighty-six thousand blister pearls and four 

 thousand free pearls have been obtained from the abalone fishermen. 



The origin of pearls has been a matter for speculation during many 



