2o6 



THE ORIGIN AND EVOLUTION OF LIFE 



marine Protozoa, skeletons of which were depositing the great 

 chalk beds of Europe and of western North America. 



The Plesiosaurs had begun their invasion of the sea during 

 Upper Triassic time, as shown in the primitive half-lizard 



Fig. 83. North America in Upper Cretaceous Time. 



The great inland continental sea extending from the Gulf to the Arctic Ocean, was favor- 

 able to the evolution of the mosasaurs, plesiosaurs, and giant sea turtles (Airhelon). 

 This period is marked by the greatest inundation of North America during Mesozoic 

 time, by mountains slowly rising along the Pacific coast from Mexico to Alaska, and by 

 volcanic activity in Antillia. Detail from the globe model in the American Museum by 

 Chester A. Reeds and George Robertson, after Schuchert. 



Lariosaurus, discovered in northern Italy, which still retains 

 its original lacertilian appearance, due to the fact that the 

 limbs and feet are not as yet transformed into paddles. In 

 the subsequent evolution of paddles the number of digits re- 

 mains the same, namely, five, but the number of the phalanges 

 on each digit is greatly increased through the process known 

 as hyperphalangy, an example of the numerical addition of 



