102 GEOLOGY. 



for flight, their heads bird-like, and their jaws set with teeth; but tooth- 

 less forms at length appeared. They were not adorned with feathers, 

 but provided with membranes stretched, in bat-like fashion, from the 

 fore limbs to the body and hinder limbs, and serving as organs of 

 flight (see Fig. 375). The fifth, or as some paleontologists believe, 

 the fourth, digit was greatly extended, and served as the chief sup- 

 port for the wing membrane. The sternum was greatly developed, 

 implying that they had true powers of flight, a conclusion sup- 

 ported by the occurrence of their remains in marine sediments free 

 from land relics, indicating burial far out to sea. They had a singu- 



Fig. 375. — Rhamphorynchus phyllurus. (Restored by Marsh.) 



larly elongated rod-like tail, with a rudder-like expansion at the end 

 (Fig. 375). 



The pterodactyls (Fig. 376) 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 unknown. 



The appearance of true birds. — A less bizarre, but really greater 

 evolution, was the contemporaneous differentiation of true birds, which 

 appeared in a similarly advanced state of development. The ancestors 

 of the pterosaurs and the birds may doubtless have been closely allied 

 far back toward the point of common saurian or stegocephalian diver- 

 gence, but there is no evidence whatever that the pterosaurs developed 

 into true birds. The two are types of analogous and parallel evolu- 

 tion, and not of successive relationship. The earliest known bird, 

 Archceopteryx macrura (Fig. 377), shows an advanced state of evolution, 

 and at the same time clear traces of reptilian ancestry. From this 



