126 THE WORLD'S LUMBER ROOM. 



transparent skin, or membrane, called the " mantle," which 

 covers the body more or less, and shapes and colours the 

 shell according to its own peculiarities. 



The Rhine alone carries to the sea every year carbonate 

 of lime enough, according to Bischof, to make the shells 

 of 332.539,000,000 oysters, and as it empties itself into 

 the German Ocean, no doubt the oyster-beds on the coast 

 of Kent are supplied with their house-building materials 

 in part at least by its means materials which have been 

 collected for the purpose far away in Germany and Swit- 

 zerland. 



Oysters are full grown in about four years, and it is 

 said that in that time, in order to obtain enough material, 

 ten of them must swallow from 345 to 587 Ibs. of sea-water, 

 or from 5-2 to 8-9 cubic feet; but this is supposing they 

 would extract the whole of the lime, and as that is unlikely, 

 they must actually swallow much more. 



The artificial beds at Whitstable and Faversham, in 

 Kent, alone extend over nearly twenty-seven square miles, 

 and the natural oyster-beds in America, some of them, cover 

 a million acres ; but it is quite beyond the power of 

 figures to express the number of oysters contained in such 

 beds, for it takes 1,600 to fill a bushel measure. One 

 oyster might, it is said, have offspring enough to fill 12,000 

 barrels.* 



Numerous beyond counting, however, as are the oysters, 

 mussels, whelks, and periwinkles, they are mentioned first, 

 not as being the chief consumers of lime, but because they 



* Five million dollars' worth are annually consumed in New York 

 alone. 



