THE JURASSIC PERIOD 723 



type of biconcave vertebrae, became differentiated into several 

 branches. Primitive lizards were doubtless abundant, but because 

 of their terrestrial habits and small size, they have little represen- 

 tation'among the fossils, and none have been found in our continent. 

 It has already been noted that crowding on the land may have 

 led some land reptiles to take to the sea. The same influence may 

 have forced others to take to the air, thereby escaping the monsters 

 of the swamps, jungles, and forests. Whatever the cause, a unique 

 feature of the period was the development of pterosaurs, or flying 



Fig. 495. Rhamphorhynchus phyllurus, a flying saurian. (Restored by 



Marsh.) 



reptiles. Appearing at the very close of the Trias in a few yet im- 

 perfectly known forms, they presented themselves at the opening 

 of the Jurassic period as fully developed flying animals (Dimorpho- 

 dori), and later formed a diversified group embracing long-tailed (as 

 Rhamphorhynchus, Fig. 495) and short-tailed forms (as Pterodactylus, 

 Fig. 496). With little doubt they sprang from some agile, hollow- 

 boned saurian, more or less remotely akin to the slender, leaping 

 dinosaurs. Between the ponderous brontosaurs (Fig. 493) and the 

 pterosaurs (Fig. 495), the Jurassic saurians present the strangest 

 contrasts. 



The Jurassic pterosaurs were small, but their successors attained 

 a wing-spread of nearly a score of feet. They were curiously com- 

 posite in structure and adaptation. Their bones were hollow, their 

 fore limbs modified for flight, their heads bird-like, and their jaws 

 set with teeth, though toothless forms appeared later. They were 



