I ON A PIECE OF CHALK 19 



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 Bracliiopoda ; 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 



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