PALEONTOLOGY. 275 



from the great phenomena of the metamorphism of organic 

 life,* have led, through the admirable labors of Deshayes and 

 Lyel], to the most marked results, especially with reference to 

 the diiierent groups of the tertiary formations, which contain 

 a considerable number of accurately investigated structures. 

 Agassiz, who has examined 1700 species of fossil fishes, and 

 who estimates the number of living species which have either 

 6een described or are preserved in museums at 8000, expressly 

 eays, in his masterly work, that, " with the exception of a few 

 small fossil fishes peculiar to the argillaceous geodes of Green- 

 land, he has not found any animal of this class in all the tran 

 sition, secondary or tertiary formations, which is specifically 

 identical with any still extant fish." He subjoins the im- 

 portant observation " that in the lower tertiary formations, 

 lor instance, in the coarse granular calcareous beds, and in the 

 London clay,t one third of the fossil fishes belong to wholly 

 extinct families. Not a single species of a still extant family 

 is to be found under the chalk, while the remarkable family 

 of the Sauroidi (fishes with enameled scales), almost allied 

 to reptiles, and which are found from the coal beds — in which 

 the larger species lie — to the chalk, where they occur individ- 

 ually, bear the same relation to the two families (the Lepi- 

 dosteus and Polypterus) which inhabit the American rivers 

 and the Nile, as our present elephants and tapirs do to the 

 Mastodon and Anaplotheriun of the primitive world. "$ 



The beds of chalk W'hicli contain two of these sauroid fishes 

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

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

 eries to consist of microscopic Polythalamia, many of which 

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

 C5ea and Baltic. The first group of tertiary formations above 

 Uie chalk, which has been designated as belonsfinof to the 

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

 since " the dawn of the icorld in which we live extends mujli 

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

 supposed. 'S^ 



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

 of all vertebrata, are found in the silurian transition strata, 



* Cuvier, Recherches sur les Ossemens Fossiles, t. i., p. 52-57. See, 

 also, the geological scale of epochs in Phillips's Geology, 1837, p. 1G6- 

 185. t [See Wonders of Geology, \o\. i,, p. 230.] — Tr 



X Agassiz, Poissons Fossiles, t. i., p. 30, and t. iii., p. 1-52 ; Buck- 

 land, Geology, vol. i., p. 273-277. 



$ Ehrenberg,' Ueber noch jetzt lehende Thierarten df Kreidelnldung. 

 in the Abhandl. der Berliie- Akad., 1831), s. l(J-l. 



