352 



ANIMAL BIOLOGY 



approaches more nearly that of the birds of the present 

 time. 



During the age when the reptiles were the dominant 

 animals on the earth we find the first remains of the highest 

 group of vertebrates, the mammals. These were repre- 

 sented for a long time by relatively small and primitive 



" 7 ~''. : ^Sffii ^'B^'s^^rr, 



FIG. 241. The Ichthyornis, a toothed bird of the Cretaceous period. 

 (After Marsh.) 



forms allied to the present-day marsupials. It was not 

 until more recent times in the Tertiary period, often called 

 the age of mammals, that the mammals became abundant. 

 They replaced the large reptiles of the previous period, 

 and their remains are in some cases sufficiently abundant 



