MOLLUSCS. 



265 



feet high, and then left to die and decompose. When the flesh is pretty thoroughly 

 disintegrated, it is washed away with water, great care being taken that none of tlie 

 pearls loose in the flesh are lost. When the washing is concluded, the shells them- 

 selves are examined for pearls, which may be attached to the interior of the valves. 

 The loose pearls are the most valuable, as they are round and moi'e apt to be free 

 from defects. Those attached to the shells have to be removed by clipping, and as 

 one side is thus defective, they can only be used in settings. For over two thousand 

 years this pearl fishery has been carried on in this place, and the result is that the shell 

 heaps are perfectly enormous, miles of territory being buried to an average depth of 

 about four feet. 



Concerning the fishery in other localities but little has been written, although 

 Panama, the island of Margarita, and the Sulu islands produce considerable numbers. 

 The best pearls are usually about the size of a pea, but the largest known was two 

 inches in length and four in circumference, and weighed three and three fourths 

 ounces troy weight. 



The pearl oyster is valued not only for the pearls which it produces, but for 

 the mother-of-pearl as well. Of this there are three varieties recognized in the 

 trade, the best of which are the silver-lipped, from the South Seas; next come the 

 black-lipped, from Manilla and Ceylon ; and lastly those known as bullock shells, from 

 Panama, etc. These last are smaller and thicker than the others. Reliable statistics 

 of the amount of the trade are difficult to obtain, but its extent may be seen from the 

 fact that Great Britain uses annually about three thousand tons, valued at half a mil- 

 lion dollars. Mother-of-pearl is used for inlaying, knife handles, etc., but the greatest 

 consumption is in the manufacture of pearl buttons. 



Mother-of-pearl is but the nacreous shell of the pearl oyster, which has an iridescent 

 appearance, due to the fine striss caused by the undulating layers of which it is com- 

 posed. The true pearls are, like the shell itself, produced by the mantle, and owe 

 their beauty to the same cause. They are, however, abnormal products, caused by 

 the deposition of the nacre around some foreign object. This nucleus may be a bit of 

 sand, a parasite, or some similar ob- 

 ject, but it is said that usually it is 

 an egg which has failed to develop 

 properly. Other forms than the 

 pearl' oyster {Meleagrina) form 

 pearls of value, while almost all 

 bivalves occasionally secrete simi- 

 lar bodies ; but, owing to the fact 

 that these partake of the nature of 

 the shell, they have not the beauty 

 of those produced by molluscs with 

 nacreous interiors. Of some of 

 these other pearls we shall have 

 occasion' to speak further along 

 when treating of some of the other 

 families. 



The genus Malleus includes the 

 hammer shells. With their long, winged hinge, and their still longer valves at right 

 angles to the hinge, they well deserve both their common and their scientific names. 



Fig. 215.— Malleus vulgaris, hammer shell. 



