been found in the rocks. The shells of this family are very broadly 

 rounded, and the hinge line is long and straight. The thickness of the 

 animal is small compared to its breadth. The pearl oyster, Melea- 

 grina margaritifera, is the most important member of the family, furnish- 

 ing, as it does, the beautiful pearls of commerce. These animals are 

 found at Madagascar, Ceylon, and other parts of the Indian Ocean. 

 The pearls are formed by some irritating substance, as a grain of sand, 

 some parasite, or even an egg getting in between the shell and the 

 animal, or lodging in some soft part, which causes the animal to cover 

 it with pearly matter to prevent irritation. Pearl gathering, by diving, 

 has been carried on at Ceylon since the time of Pliny, and several 

 remarkable pearls have been found. The largest pearl known, that of 

 Mr. Hope, weighed eighteen thousand grains, and measured two inches in 

 length and about four inches in circumference. The Chinese produce arti- 

 ficial pearls, as well as pearl images, 

 by inserting a lead figure, or small, 

 round object between the mantle and 

 the shell, which the animal covers 

 with pearl in a few months. The shells 

 of the pearl oyster furnish the larger 

 part of the mother-of-pearl, which is 

 so largely used for ornamental pur- 

 poses, and several hundred tons are 

 imported into England annually, prin- 

 cipally from Manila. A species of 

 pearl oyster lives at Panama, and a 

 related species, Pteria radiata, in 

 Florida and the West Indies. 



" You will probably remember," said Professor Parker, stepping over 

 to another case, " the large bank of mussel shells which you saw on 

 our trip to the beach at Rocky Point. In this case are exhibited 

 a large variety of these shells. Notice the peculiar, wedge-shaped 

 shell, with its small, rounded anterior end, and broad, sharp posterior 

 end. Note also the little bunch of thread-like filaments which extend 

 from near the anterior end of the shell, and fastens that large speci- 

 men to a stone. That is the byssus, which is spun by the foot of the 

 animal, and by which they attach themselves to mud banks and vege- 

 tation. The huge banks of these mussels, some of which have black, 

 glossy shells, are familiar objects to the seaside visitor. At low water 

 they are among the most conspicuous objects on the shore. A related 

 genus, Modiola, burrows in the ground or spins a nest formed of stones 



7G 



Malleus vulgaris, the " hammer - head 

 oyster" of the Indian Ocean. A near rela- 

 tive of the pearl oyster. (Tryon.) 



