212 OYSTERS, AND ALL ABOUT THEM. 



The great mass of the chalk is composed, as we have 

 seen, of the skeletons of the Globigerinag, and other simple 

 organisms, imbedded in granular matter. Here and there, 

 however, this hardened mud of the ancient sea reveals the 

 remains of higher animals which have lived and died, and 

 left their hard parts in the mud, just as the oysters die and 

 leave their shells behind them, in the mud of the present 

 seas. 



There are, at the present day, certain groups of 

 animals which are never found in fresh waters, being 

 unable to live anywhere but in the sea. Such are the 

 corals ; those corallines which are called Polyzoa ; those 

 creatures which fabricate the lamp-shells, and are called 

 Brachiopoda ; the pearly Nautilus, and all animals allied to 

 it; and all the forms of sea-urchins and star-fishes. 



Not only are all these creatures confined to salt water 

 at the present day, but, so far as our records of the past 

 go, the conditions of their existence have been the same ; 

 hence, their occurrence in any deposit is as strong evidence 

 as can be obtained that that deposit was formed in the sea. 

 Now the remains of animals of all the kinds which have 

 been enumerated, occur in the chalk, in greater or less 

 abundance ; while not one of those forms of shell-fish 

 which are characteristic of fresh water has yet been 

 observed in it. 



When we consider that the remains of more than three 

 thousand distinct species of aquatic animals have been 

 discovered among the fossils of the chalk, that the great 

 majority of them are of such forms as are now met with 

 only in the sea^ and that there is no reason to believe that 

 any one of them inhabited fresh water, the collateral 

 evidence that the chalk represents an ancient sea-bottom 



