562 HISTORICAL GEOLOGY 



this way lessened. This adaptation to aquatic conditions may have 

 been so perfect that not only flying, but walking as well, was aban- 

 doned. The bird was covered with soft feathers, as fossil impressions 

 show. Hesperornis lived only in the Upper Cretaceous. 



Fig. 522. — Hesperornis, a diving, toothed bird of the Cretaceous. (Restoration 

 under the direction of F. A. Lucas.) 



Ichthyornis (Greek, ichthus, fish, and ornis, a bird). — This bird 

 (Fig. 523) was about as large as a pigeon and must have looked very 

 much like a modern bird. It was, however, radically different in some 

 particulars. Its slender jaws were toothed, the teeth being small 

 and set in sockets, twenty on each side below and fewer above. The 

 vertebrae were biconcave, like those of fishes and many extinct reptiles 

 but no modern bird. The tail was about midway between the verte- 

 brated tail of Archaeopteryx and those of the birds of the Tertiary and 

 to-day. The strongly keeled breastbone for the attachment of the 

 muscles proves that it was a powerful flyer. Although Ichthyornis 

 shows a distinct advanceover Archaeopteryx in its less vertebrated tail, 

 its power of flight, and the loss of the claws on the fore limbs, an equal 

 or greater change is to be seen between the Cretaceous birds and tnose 

 of the Tertiary. 



It is interesting to speculate upon the cause of the abandonment 

 of teeth for a horny jaw both by birds and pterosaurs. A toothed 



