182 



GEOLOGY. 



The latter began in the Laramie epoch, and continued into the Eocene; 

 the former is known only from the Lower Eocene 



The sea birds. — In the long interval between the first known appear- 

 ance of birds in the 4 Jurassic, and the later Cretaceous, when they re-ap- 

 peared, important changes took place, among which was the loss of 

 the elongate, bilaterally feathered tail. The Jurassic birds were ter- 



Fig. 415. — Hesperornis regalis. Skeleton in U. S. National Museum from which 

 the restoration Fig. 415a was made. Sternum and two anterior cervicals sup- 

 plied by restoration. (Lucas.) 



restrial, while the Cretaceous were aquatic. The Cretaceous birds 

 belonged to two widely divergent classes, the one consisting of large 

 flightless birds (Hesperornis), the other of small birds of powerful 

 flight (I chthyornis) . The Hesperornis (Fig. 415), was a large, flightless, 

 highly specialized diver, with aborted wings and remarkable leg develop- 

 ment. The wings had almost vanished, a single bone only being left. 

 This implies that, following the evolution which had produced the 

 wings, there was a degenerative history long enough for them to dwindle 



