88 MYA. 



which they protrude above the surface of the sand, and 

 through which they respire and take their food. The 

 animal of the M. Pictorum, on the contrary, has no such 

 provision ; it has merely a short muscular foot, which it 

 protrudes from the hinder part of the shell, and is the 

 organ by which it moves its habitation from one place 

 to another. The shell is generally found buried in sand 

 or mud ; and the animal, though an inhabitant of the 

 water, is capable of existing even when deprived of its 

 native element. Bosc observed them alive, in America, 

 in hardened mud, which resisted his efforts with a 

 spade to dig them out ; it seems they had remained in 

 this situation during three or four summer months, 

 without any refreshment except a few slight showers. 



One of the species of this genus, the M. margari- 

 tifera, has long been celebrated for the pearls which it 

 has at different times produced. It is upon record, in 

 the second volume of the Philosophical Transactions 

 Abridged, that several pearls of great size were procured 

 from this shell, in the rivers of the county of Tyrone 

 and Donegal, in Ireland. One that weighed thirty-six 

 carats was valued at forty pounds, but being foul, lost 

 much of its worth. Other single pearls were sold for 

 four pounds ten shillings, and ten pounds ; and it is de- 

 serving of notice, that the last was sold a second time 

 to Lady Glenlealy, who put it into a necklace, and re- 

 fused eighty pounds for it from the Duchess of Ormond. 



The pearl is produced by a deposition of testaceous 

 matter on the interior coat of the shell ; it is a tubercle 

 formed at pleasure by the animal, and is generally 

 secreted to repair the damages made by certain worms 

 that penetrate the inside of the shell near the edge of 

 the valves, and, working themselves a passage between 



