PALJSOZOOLOGY : DOSSIL ANIMALS. 265 



(the Lepidosteus and the Polypterus), similar to those exhi- 

 bited by the Mastodon and Anoplotherium of the ancient 

 world when compared with our elephants and tapirs. ( 307 ) 



We learn from Ehrenberg's remarkable discoveries, that 

 the beds of chalk, which still contain two species of these 

 sauroid fishes, and in which are found gigantic reptiles, and 

 a considerable number of corals and shells which no longer 

 exist, are nevertheless composed in great part of microscopic 

 polythalainia, many of which are still living even in the seas 

 of our own latitudes, in the North Sea, and in the Baltic. 

 Thus we see that the tertiary, group of deposits resting imme- 

 diately upon the chalk, and to which the name of Eocene group 

 is usually given, is not strictly entitled to that desig- 

 nation, for the dawn of the world in which we live extends 

 much farther back in the history of our planet than has 

 been hitherto supposed. ( 308 ) 



We have seen that fishes, which are the oldest verte- 

 brated animals, first appear in the silurian strata, and 

 are found in all the succeeding formations up to the 

 beds of the tertiary period. Reptiles begin in like manner in 

 the zechstein (magnesian limestone) ; and if we now add, that 

 the first mammalia (the Thylacotherium prevostii and Thyla- 

 cotherium bucklandi, allied, according to Valenciennes, ( 309 ) 

 to marsupial animals,) are met with in the Stonesfield 

 slate, a member of the oolite, and that the first remains of 

 birds have been found in the deposits of the cretaceous 

 period, ( 31 ) we shall have indicated the inferior limits, 

 according to our present knowledge, of the four j?reat divi- 

 sions of the vertebrata. 



In regard to invertebrate animals, we find corals and 

 some shells associated, in the oldest formations, with very 



