82 TESTACEA. 



animals are thick, fleshy, of good colour, and fine flavour. But I could 

 not discover any sensible diflerence, either in them or their shells, from 

 the ordinary species. Indeed, I have recently understood that Lord 

 Tankerville'.s oyster bed has been recruited from Prestonpans. The 

 largest of the shells might be circumscribed by a circle of four inches and 

 a half in diameter. They are tolerably clean, except for a few small 

 marine animal or vegetable matter. The bed is described as very small, 

 not extending above 200 yards : covered by about six inches of water at 

 ebb tide. 



On the whole, I am satisfied that their size and quality depend 

 entirely on their site. 



There is an extraordinary disproportion in the quantity of calcareous 

 matter secreted and deposited by the different species of Testacea, espe- 

 cially by the bivalves, which are the more stationary. Sometimes the 

 shell will be found many times the weight of the animal. 



There is another product of the bivalves, concerning which some- 

 thing might be said, the origin of pearls. Small specimens are found in 

 both the mussel and the oyster ; but especially in the fresh water shells 

 usually associated with the former genus. 



The Scotish pearls have borne considerable repute during centuries : 

 they are alluded to repeatedly in history, as recovered from the shells of 

 various rivers, in the north, the west, and the south. And the traveller 

 may often observe quantities cast up on their bank.s, by adventurers in 

 quest of their precious contents. Traders, also, sometimes bring their 

 collections to the city of Edinburgh for sale. If memory serves me, the 

 late Mr John Pinkerton had collected a number, among which he prized 

 black pearls the most. One of the Scotish pearls is said to be now an 

 ornament of the imperial (n-own. 



Much has been said and written on the formation of pearls, — which, 

 indeed, is a very interesting subject. Naturalists ha\'e patiently awaited 

 for years the result of experiments designed to generate them. 



Very few naturalists seem to have devoted their special investiga- 

 tions to the nature of the animals inhabiting shells. Undoubted^, the 



