278 COSMOS. 



as our present elephants and tapirs do to the Mastodon and 

 Anaplotheriun of the primitive world."* 



The beds of chalk which contain two of these sanroid fishes 

 and gigantic reptiles, and a whole extinct world of corals and 

 muscles, have been proved by Ehrenberg's beautiful discove- 

 ries to consist of microscopic Polythalamia, many of which 

 still exist in our seas, and in the middle latitudes of the North 

 Sea and Baltic. The first group of tertiary formations above 

 the chalk, which has been designated as belonging to the 

 Eocene Period, does not, therefore, merit that designation, 

 since " the dawn of the world in which we live extends much 

 further back in the history of the past than we have hitherto 

 supposed."! 



As we have already seen, fishes, which are the most ancient 

 of all vertebrata, are found in the Silurian transition strata, and 

 then uninterruptedly on through all formations to the strata 

 of the tertiary period ; whilst Saurians begin with the zech- 

 stone. In like manner we find the first mammalia (Thyl(b- 

 cotherium prevostii, and T. bucklandii, which are nearly allied, 

 according to Valenciennes,^; with marsupial animals,) in the 

 oolitic formations (Stonesfield- schist), and the first birds in the 

 most ancient cretaceous strata. Such are, according to the 

 present state of our knowledge, the lowest!! limits of fishes, 

 saurians, mammalia, and birds. 



Although corals and serpulidse occur in the most ancient 

 formations simultaneously with highly developed Cephalo- 

 podes and Crustaceans, thus exhibiting the most various orders 

 grouped together, we yet discover very determinate laws in 

 the case of many individual group? of one and the same orders. 



* Agassiz, Poisso nsfossiles, t. i. p. 30, and t. iii. pp. 1-52 ; Buckland, 

 Geology, vol. i. pp. 273-277. 



*t* Ehrenberg, Ueber nodi jetzt lebende Thierarten der Kreidebil- 

 dung, in the Abhandl. der Berliner AJead., 1839, s. 164. 



J Valenciennes, in the Comptes rendus de VAcademie des Sciences, 

 t. viL, 1838, pt. 2, p. 580. 



In the Weald-clay; Beudant, Geologic, p. 173. The ornitholites 

 increase in number in the gypsum of the tertiary formations Cuvier, 

 Ossements fossiles, t. ii. p. 302-328. 



|| [Recent collections from the southern hemisphere, show that this 

 distribution was not so universal during the earlier epochs, as has gene" 

 rally been supposed. See Papers by Darwin, Sharpe, Morris, and 

 McCoy, in the Geological Journal.] Tr. 



