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 Linnzeus 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 fromthe Avicula, being usually 
tinged with brown. But the shell which in Britain produces 
the finest pearls, is the Alasmodon margaritiferum, which was 
placed by Linnzeus 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 
