MOLLUSCA. 503 



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 the 

 Unio margaritifera, 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 Ythan in Scot- 

 land, have long been famous for the production of pearls. 

 These concretions are found between the membranes of the 

 cloak of the animal, as in the Avicula, or adhering to the 

 inside of the shell, as in the Unio. In the former case, they 

 seem to be a morbid secretion of testaceous matter ; in the 

 latter, the matter appears to be accumulated against the in- 

 ternal opening of some hole with which the shell has been 

 pierced by some of its foes. Linn.eus, from the con- 

 sideration of this circumstance, endeavoured, by piercing 

 the shell, to excite the animal to secrete pearl ; but his at- 

 tempts, though they procured him a place among the Swe- 

 dish nobility, and a pecuniary reward, were finally aban- 

 doned ; the process being found too tedious and uncertain 

 to be of any public utility. The largest pearl of which we 

 have any notice, is one which came from Panama, and was 

 presented to Philip II. of Spain, in 1579. It was of the 

 size of a pigeon"'s egg. Sir Robert Sibbald mentions his 

 having seen pearls from the rivers of Scotland as large as a 

 bean. 



The reproductive organs of the Bivalvia, hitherto exam- 

 ined, consist of an ovarium occupying the sides of the body, 

 and penetrating the membranes of the cloak. They appear 

 to have the organs of both sexes incorporated, and to pro- 

 pagate without intercourse. Lamarck is disposed to con- 

 sider impregnation as produced by the male fluid dispersed 

 through the water ; a supposition unsupported even b)' 



