LATER LIFE ON THE EARTH 51 



attained the enormous length of seventy feet. They 

 were Cretaceous only. The Ornithosauria , or flying 

 reptiles, lived in the Jurassic and Cretaceous periods. 

 They stand apart from other reptiles, showing only 

 very slight relations to the Anomodontia. They 

 have some bird-like characters such as a keeled 

 sternum and hollow bones but these are only adap- 

 tive, and essential for flight; they show no real 

 relationship to birds. The skull also looks some- 

 thing like that of a bird, but it is really more like 

 that of a lizard. 



All the Dinosaurians lived on land, and some of 

 them were the largest of land animals, being over a 

 hundred feet in length. They differed greatly in 

 appearance ; for, while the earlier forms had the 

 shape of crocodiles, to which they were closely re- 

 lated, others resembled in shape the rhinoceros, and 

 others again the kangaroo ; for they walked on their 

 hind feet, which had only three toes, and their fore- 

 legs were sometimes small. In some of these, the 

 bones of the pelvis make a considerable approach to 

 that of birds; and Professor Marsh has described, 

 from the American Jurassic rocks, some small Dino- 

 saurians, apparently closely related to birds, which 

 he thinks may have been arboreal in their habits. 

 ' The difference," he says, " between them and the 

 birds that lived with them may have been at first 

 mainly one of feathers." 11 The early Crocodiles are 

 difficult to distinguish from Dinosaurians, but after- 

 wards they become more specialised ; and there is 

 abundant evidence to show that the modern croco- 

 u American Journal of Science, vol. xxii. (1881), p. 340. 



