516 THE JURASSIC PERIOD 



orous forms; but among them all there was not a single type which 

 was distinctively North American. It is therefore concluded that 

 there was freedom of migration between the eastern and western 

 continents at this time. 



Of the carnivores, one of the most common was a type (Fig. 439) 

 whose fore limbs seem to have been used chiefly for seizing and hold- 



Fig. 439. A carnivorous dinosaur, Ccratosaurus nasicornis, about 1/40 natural 

 size; i.e., length about 17 feet; from the Como beds, Colorado. (Restoration of 

 skeleton by Marsh.) 



ing prey, rarely for walking. The animal's pose was facilitated by 

 hollow bones. The head was relatively large, an unusual character 

 for a race among which small heads and brains were the fashion. 

 Besides the large ones, there were small leaping forms not larger than 

 a rabbit. 



The herbivorous dinosaurs, known first in this system, developed 

 so rapidly that they soon outranked the carnivorous forms in both 

 size and diversity. Most of them were massive, with sub-equal 

 limbs and the quadrupedal habit. Some of them (Fig. 440) attained 

 a length of 60 feet (possibly more), taking rank among the largest 

 of known land animals. These enormous creatures were char- 

 acterized by weakness rather than strength, for they were unwieldy, 

 their heads and brains small. "The task of providing food for so 



