STRUCTURE OF THE VERTEBRATES 351 



sic, contemporaries of the dinosaurs. The earliest mammals were 

 about the size of mice. When the first Jurassic mammalian jaw 

 (Amphitherium) was discovered more than a century ago Cuvier 

 called it a small opossum, and little evidence has since appeared 

 to change the classification. In the Cretaceous appear numerous 

 definite marsupials, roughly contemporary with the early In- 

 sectivora. 



In the earliest strata of the Paleocene there is a fullblown 

 mammalian fauna. These were all small, generalized, and dom- 

 inantly placental. From the insectivorous stock had developed 

 two other of the living orders, the carnivores and the primates. 

 The latter were apparently herbivorous or omnivorous. Tlie pri- 

 mates of the Paleocene were lemuroid in structure, and from 

 this group during the succeeding periods evolved the lemurs, 

 monkeys, anthropoids and man. 



During Eocene time most of the land mammals of the present 

 time had their beginning. On the plains were the four-toed (and 

 later three-toed) ancestors of the horse. The leathery armadillos 

 had attained fair size; and the Artiodactyla, the even-toed ungu- 

 lates, had begun their radiations. The archaic carnivores had 

 attained larger size, these creodonts cUsappearing in the Oli- 

 gocene Period, their smaller and more generalized relatives sur- 

 viving as the present Carnivora. In this period arose the largest 

 of the early mammals, the herbivorous, horned, Titanotheres. 



The following succession of strata show the rise of hundreds 

 of genera, and a number of orders. Eleven of the known orders 

 completely disappeared in the struggle for existence, although 

 sixteen orders have survived to the present. These surviving 

 orders have become adapted to nearly every type of environ- 

 ment. For their adaptive radiations the student is referred to 

 Scott's A History of the Land Mammals in the Western Hem- 

 isphere. 



G. Evolution of Man 



The first primates appeared in the Paleocene Period, are 

 lemuroid in structure, and appeared almost synchronously in 

 Europe and America. All were adapted for arboreal life. The 

 orbits were large, indicating large and (by analogy with the 



