36 MOLLUSC A, 



way ; and is also very interesting on account of the 

 sparkling aspect of the double row of bead-like eyes 

 and the beauty of the mantle fringes. 



The Pearl Oyster, or Meleagrina margaritifera, are 

 often obtainable in curiosity shops or natural history 

 stores. Mother of Pearl is the nacreous layer taken from 

 the inner side of this shell, though other shells also fur- 

 nish a proportion of this article, — the inside layer be- 

 ing usually more or less nacreous. Pearls themselves 

 are pathological products ; and, though common enough 

 among the oysters, not very perfect or valuable when 

 obtained from other forms than the Pearl Oyster. 



Imperfect pearls are common in oysters, and in the 

 fresh-water clams of our western rivers ; these last are 

 sometimes quite pretty, though very small and irregu- 

 lar. They often have a nucleus of sand or some 

 foreign matter which has lodged between the mantle 

 and the shell. This causes an extraordinary excretion 

 of shelly matter to take place, which ultimately builds 

 itself around the original nucleus in a large number of 

 concentric coats, like those of an onion. Pearls, there- 

 fore, are easily injured and dissolved by acids, being 

 of the same structure as the nacreous layers. 



The most ancient and successful fisheries are in the 

 Red Sea and off Ceylon, though this species occurs 

 and is profitably hunted for its pearls and shell also at 

 the Society Islands, at Manilla, at Panama, and many 

 other places in tropical latitudes. 



It lives on the bottom, in about twelve fathoms, and 

 is gathered by diving. 



