766 



GEOLOGY 



and perhaps the direct ancestors, of the mosasaurians (Fig. 518), 

 a family which flourished in the Cretaceous, and ranged from 

 North and South America to Europe and New Zealand. Their 

 short career seems to have ended with the period, and no direct 

 descendants are known. 



Fig. 518. A Cretaceous mosasaurid, Platacarpus coryphceus Cope, restored 

 by Williston; from Upper Cretaceous, Kansas. 



Marine turtles seem to have appeared first in this period, and 

 have deployed into many and diverse forms. The largest we 

 broad, and flat, degenerate in having the carapace reduced to the 

 ribs alone, and probably covered with a soft skin, like some living 

 marine turtles. Some of them had skulls larger than those of horses 

 and must have measured fully twelve feet across the shell. 



: 



Fig. 519. Champsosaurus, from the Laramie of Montana. Length, about 

 six feet. (After Brown.) 



In the long interval between the first known appearance of />//v/x 

 in the Jurassic, and the later Cretaceous when they reappeared, 

 important changes took place, among which was the loss of the 

 elongate, bilaterally feathered tail. The Jurassic birds were terres- 

 trial, while the Cretaceous were aquatic. The Cretaceous l>i id- 

 include about 30 species belonging to two widely divergent orders. 

 Hesperornis and Ichthyornis. The former (Fig. 520) were l:i 



