4:26 ANIMAL STUDIES 



In a later epoch the batrachians or amphibians appeared ; 

 in a still later period, the reptiles ; and last of all, the birds 

 and the mammals, the last being the highest of the back- 

 boned animals. On the opposite page is shown a table 

 giving the names and succession of the various geological 

 periods, and indicating briefly some of the kinds of animals 



FIG. 248. A Crossopterygian ft$h(0steolepis macrolepidotits), Devonian of Scotland. 

 From Zittel, after PAUDER. 



living in each. In each of these divisions of geological 

 time some one class of animals was especially 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 De- 

 vonian, as the age of fishes. In the same way we have the 



FIG. 249. Cladosdache fyleri (Newberry). After DEAN, from Devonian rocks in Ohio. 

 The most primitive of known sharks. 



Reptilian Age, the Mammalian Age, according to the great 

 class of animals predominating at that time. Of course, in 

 each of the later epochs there lived animals representing 

 the principal classes or groups in all of the preceding ones, 

 as well as the animals of that particular group which may 

 have first appeared in this epoch, or was its dominant group. 



