2IO THE ORIGIN AND EVOLUTION OF LIFE 



propulsion is not chiefly by means of the fins but by the sinu- 

 ous motions of the body, and especially of the very elongate, 

 broad, fin-like tail. These sea lizards of Upper Cretaceous 

 time (Fig. 76) are analogous or convergent to the sea Croco- 

 dilia (Geosaurus) of Jurassic time and present further analogies 

 with the Triassic ichthyosaur Cymbospondylus and the small 

 Permo- Carboniferous amphibian Cricotiis (Fig. 76), In the 

 American continental seas these animals radiated into the 

 small, relatively slender Clidastes, into the somewhat more 

 broadly finned Platecarpus, and into the giant Tylosaurus, 

 which was capable (Fig. 87) of capturing the great fish of the 

 Cretaceous seas (Porlheus). 



Terrestrial Life. Carnivorous Dinosaurs 



Widely contrasting with these extreme adaptations to 

 aquatic marine life, the climax of terrestrial adaptation in the 

 reptilian skeleton is reached among the dinosaurs, a branch 

 which separated in late Permian or early Triassic time from 

 small quadrupedal, swiftly moving, lizard-like reptiles and 

 before the time of their extinction at the close of the Creta- 

 ceous had evolved into a marvellous abundance and variety 

 of types. In the Upper Triassic of North America, late New- 

 ark time, the main separation of the dinosaurs into two great 

 divisions, (a) those with a crocodile-like pelvis, known as 

 Saurischia, and (b) those with a bird-like pelvis, known as Orni- 

 thischia, had already taken place, and the dinosaurs domi- 

 nated all other terrestrial forms. 



When Hitchcock in 1836 explored the giant footprints in 

 the ancient mud flats of the Connecticut valley he quite nat- 

 urally attributed many of them to gigantic birds, since at the 

 time the law of parallel mechanical evolution between birds 

 and dinosaurs was not comprehended and the order Dino- 



