58 INTRODUCTIOX. 



The situation in oui' systems which has been 

 allotted to fishes, has generally been the fourth 

 place, or the lowest rank, in the scale of the ver- 

 tebrata. They seem to have been more particu- 

 larly connected with the class which immediately 

 precedes them, by those most extraordinary crea- 

 tures, long since extinct, but which now occupy 

 so much of the attention of the geologist, the 

 Ichthyosauri or Fish-Lizards, and which the dis- 

 covery of new forms lead us to believe were a 

 numerous race, perhaps possessing intermediate 

 ability to exist either in air or water. Fishes are 

 entirely inhabitants of the waters, peopling this 

 immense portion of our globe with their shoals, and 

 serving to keep in check the varied creatures of 

 still lower structure, while they themselves are 

 held in check, and afford sustenance to millions 



fourths of this number belong to two only of the above men- 

 tioned orders, the Cycloidians and Ctenoidians, whose pre- 

 sence has not been discovered in formations below the chalk. 



The other fourth part of living specips is referable to the 

 orders Placoidians and Ganoidians, which are now far from 

 numerous, but which existed during the whole period 

 which elapsed since the earth began to be inhabited, to the 

 time when the animals of the Greensand lived. 



M. Agassiz does not know a single species of fossil fish 

 which is found successively in two formations, while he is 

 acquainted with a good number which have a very consider- 

 able horizontal extent Ed. Phil. Jour. xxxv. 175. 



