142 HISTORICAL GEOLOGY 



stage of the typical modern Fishes (Teleosts) , and the vertebrated 

 character of the tail fin, the tail of the modern Teleost successively 

 passing from the asymmetric vertebrated stage, to symmetric verte- 

 brated, and finally to symmetric non-vertebrated (Fig. 84). 



Generalized or synthetic types, so well illustrated by Devonian 

 Fishes, are of great importance in considering the evolution and 

 geological history of organisms. 



Amphibians. — Footprints, considered to be those of Amphib- 

 ians, have been found in the Upper Devonian strata of Pennsyl- 

 vania, but remains or impressions of the animals are not known. 

 At any rate it seems certain that terrestrial Vertebrates existed as 

 early as the Devonian. 



