PALEONTOLOGY 



297 



visions of geological time some one class of animals was espe- 

 cially numerous in species, and was evidently the dominant 

 group of animals through that period. The different ages are 

 therefore spoken of in terms of the prevailing life. Thus, the 

 "Silurian Age" is known as the age or era of invertebrates; 

 the " Devonian/' as the age of fishes. In the same way we 

 have the "Reptilian Age/' the "Mammalian Age/' according 



