THE TRIASSIC PERIOD 



217 



Mammals. — Another very important step in the development 

 of animal life was the introduction of Mammals in the Triassic 



Fig. 131 



Tracks of a large two-legged Dinosaur on Triassic sandstone from the Con- 

 necticut Valley, showing how both feet slid some distance in the soft ma- 

 terial after which the creature suddenly sat down, the end of the backbone 

 having left a distinct impression. (After Edward Hitchcock.) 



period. Although Mammals include the most highly developed of 

 all animals, their earliest representatives (in the Triassic) were 

 very small, primitive types, apparently not very numerous, thus 



