Page:Popular Science Monthly Volume 82.djvu/547

Rh 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 thousand 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,

pebbles and other foreign bodies retained beneath the mantle. Sometimes 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, varying 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 thousand 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