THE CRETACEOUS PERIOD 425 



than at periods like the present, in which comparatively 

 rapid changes have been taking place. Uniform climates 

 permitted the migration of both animals and plants over 

 wide stretches of the continents and of the seas. The 

 broadly expanded shallow seas afforded a congenial field for 

 the increase of marine life. Along with shells, corals, and 

 other remains of sea animals, we find in the Cretaceous rocks 

 the bones of many marine reptiles, not only turtles like 



FIG. 445. A Cretaceous mosasaur. (Painted by C. R. Knight, under the 

 direction of Professor H. F. Osborn. Copyright by Amer. Mus. of Nat. 

 Hist.) 



those which now inhabit the oceans, but long, serpentlike 

 forms in which the legs were reduced to short paddles, while 

 the long, flattened tail served as a strong propeller (Fig. 445). 

 The sharp teeth of these mosasaurs is ample evidence of 

 their ferocious nature. 



Eccentric forms of the older reptiles. The large land- 

 inhabiting reptiles, or dinosaurs, which had reached the 

 zenith of their career in the late Jurassic, were still abundant, 

 but had entered upon a period of eccentric diversification 



