MOLLUSCS. 



265 



fuL't high, and tlicn left to die niid decompose. When tlie flesh is pretty thoroughly 

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

 pearls loose in the flesh are hist. 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 iiearls are the most valuable, as they are round and more 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 j)lace, and the result is that tlie 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, althiiuixh 

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

 Tlie best pearls are usually about the size of a ]>ea, but the largest known was two 

 inches in length and four in circumference, and weiglied 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 tliis there are three varieties recognized in the 

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

 black-lipped, from Manilla and Ceylon ; and lastly those kno\\n 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 lian<lles, etc., but the greatest 

 consumption is in the manufacture of peai'l buttons. 



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

 ap]iearance, due to the fine stri;e caused by the undulating layers of which it is eom- 

 jiosed. 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. 

 jiroperly. Other forms than the 

 ]iearl oyster {MeleagriiKi) form 

 jiearls 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 j)roduced 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 Malleiis 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. 



Fk:. T,<\. — Malleus vulgaris, hammer shvll. 



