PALAEONTOLOGY. 277 



Mexican lakes, belongs to the most recent fresh-water forma- 

 tions of CEningen.* 



The determination of the relative ages of organisms by 

 the superposition of the strata has led to important results 

 rding the relations which have been discovered between 

 extinct families and species, (the latter being but few in 

 number) and those which still exist. Ancient and modern ob- 

 svrvations concur in showing that the fossil floras and faunas 

 differ more from the present vegetable and animal forms in. 

 proportion as they belong to lower, that is, more ancient 

 sedimentary formations. The numerical relations first deduced 

 by Cuvier from the great pheonomena of the metamorphism 

 of organic life,f have led, through the admirable labours of 

 Deshayes and Lyell, to the most marked results, especially 

 with reference to the different groups of the tertiary forma- 

 tions, 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 been described or are pre- 

 served in museums, as 8000, expressly says, in his masterly 

 work, that " with the exception of a few small fossil fishes 

 peculiar to the argillaceous geodes of Greenland, he has not 

 found any animal of this class, in all the transition, secondary 

 or tertiary formations, which is specifically identical with any 

 still extant fish." He subjoins the important observation 

 " that in the lower tertiary formations, for instance, in the 

 coarse granular calcareous beds, and in the London clay4 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; whilst the remarkable family of the Scmroidi 

 (fishes with enamelled 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 individually, bear 

 the same relation to the two families, (the Lepidosteus and 

 Polypterus,) which inhabit the American rivers and the Nile, 



* [Ansted's Ancient World, p. 56.] Tr. 



t Cuvier, Recherches sur les Ossemens fossiles, t. i. pp. 52-57. See 

 tlso the geological scale of epochs in Phillips' Geology, 1837, pp. 166- 

 185. 



J See Wonders of Geology, vol. D. 230 1 Tr 



