TIM GEOLOGY 



provided with membranes stretched, bat-like, from the fore limbs 

 to the body and hind limbs, and serving as organs of flight (Fig. 495). 

 The fifth, or as some paleontologists believe, the fourth, digit was 

 greatly extended, and served as the chief support for the wing- 

 membrane. The sternum was greatly developed, implying true 

 powers of flight, a conclusion supported by the occurrence of their 



Fig. 496. Skeleton of pterodactyl, Pterodactylus spectabil/x, from tin- litho- 

 graphic stone at Eichstadt, Bavaria; about % natural size. (After H. 

 v. Meyer.) 



remains in marine sediments free from other land fossils. 



of them had singular elongate rod-like tails, with a rudder-like 



expansion at the end. 



The pterodactyls (Fig. 496) had short tails, and were usually 

 small and slender. Fully differentiated as first found, the ptero- 

 saurs underwent no radical change of structure during their career. 

 and the steps of their remarkable evolution are for the most part un- 

 known. Representations of flying reptiles among the Jura i' 

 fossils of North America are extremely rare. 



