226 MOLLUSCA. 



ment, and to the discovery of new species, than to the habits 

 and uses of those already known. 



Independently of the food which we thus obtain from 

 testaceous animals, they furnish us with the pearl, one of 

 the most beautiful ornaments of dress. This substance, 

 equally prized by the savage and the citizen, is composed, 

 like shells, of carbonate of lime, united with a small portion 

 of animal matter. Pearls appear to be exclusively the pro- 

 duction of the bivalve testacea. Among these, all the shells 

 having a mother-of-pearl inside, produce them occasionally. 

 But there are a few species which yield them in greater 

 plenty, and of a finer colour. The most remarkable of these 

 is the Avicula margaritifera. This shell, which was placed 

 by Linnaeus among the mussels, is very widely distributed 

 in the Indian seas ; and it is from it and another species of 

 the same genus, termed Avicula hirundo, found in the Eu- 

 ropean seas, that the pearls of commerce are procured. The 

 Pinna, so famous for furnishing a byssus or kind of thread, 

 with which garments can be manufactured, likewise pro- 

 duces pearls of considerable size. They have seldom the 

 silvery whiteness of the pearls from the Avicula, being usually 

 tinged with brown. But the shell which in Britain produces 

 the finest pearls, is theAlasmodon margaritiferum, which was 

 placed by Linnaeus in the genus Mya. It is found in all 

 our alpine rivers. The Conway and the Irt in England, the 

 rivers of Tyrone and Donegal in Ireland, and the Tay and 

 the Yythan in Scotland, have long been famous for the pro- 

 duction of pearls. These concretions are found between 

 the membranes of the cloak of the animal, as in the Avi- 

 cula, or adhering to the inside of the shell, as in the Unio. 

 In the former case, they seem to be a morbid secretion of 



