550 



THE CRETACEOUS PERIOD 



went a marked change early in the period, developing into the modern 

 forms, though some of the old types lived on. 



Flying reptiles made so distinct an advance in specialization 

 that Williston regards them as having come to excel all other flying 



Fig. 464. A Cretaceous pterodactyl, Nyctosaurus gracilis Marsh, about i/io 

 natural size, Niobrara Cretaceous, Kansas. (Restored by Williston.) 



vertebrate animals. Some had a wing-spread of perhaps 20 feet. 

 In some of the genera (Fig. 464) the development of the front parts 

 was great, while the hind parts were so very small and weak that it 

 is doubtful whether they could stand on their feet alone. The 

 Cretaceous forms were all short-tailed, and for the most part tooth- 

 less. Their bills resembled those of modern birds. 



Terrestrial birds existed, but their record is meager. There 

 were some curious aquatic forms, which will be mentioned with 

 the sea life. The mammals thus far recovered from the Cretaceous 



Fig. 465. A Cretaceous mosasaur, Platacarpus coryphceus Cope, restored by 

 Williston; from Upper Cretaceous, Kansas. 



indicate little advance on those of the Jurassic, and they appear to 

 have played very little part in the fauna of the period. 



Sea life. Vertebrates. The ichthyosaurs and plesiosaurs which 



