Williston.] Birds. 49 



articulation the end of the centrum has the surface concave in 

 one direction and convex in the other, corresponding to similar 

 but reversed concavity and convexity in the adjacent surface of 

 the contiguous vertebra?. In Ichthyornis this peculiarity was 

 almost wholly wanting, the two ends of the centrum being 

 nearly alike and gently concave in the middle. This concavity 

 is not nearly so deep as in fish vertebrae, but is nevertheless of 

 that type, which suggested the generic name from ichthyos, fish, 

 and ornis, bird. In other respects Ichthyornis did not differ 

 notably from the common flying birds of the present time. 

 Among recent birds the tern seems to approach Ichthyornis 

 most closely, due, doubtless, to similar modes of life. In pi. vn 

 will be seen the vertebra? and jaws of Ichthyornis , after Marsh. 



Because Hesperornis was a swimming bird and Ichthyornis a 

 bird of powerful flight, skimming over the waters after the 

 manner of the petrel, they have been more subject to fossiliza- 

 tion than the strictly land-inhabiting birds were. Certainly 

 there were many other species and genera of birds in existence 

 at the time when these lived, since the great difference between 

 the two forms could not have been attained without the devel- 

 opment of many other forms. Of these, however, we have very 

 few or no remains. AVhether all birds contemporary with them 

 were toothed or not it is impossible to say, but the probability 

 is that they were. 



In pi. v a restoration of Hesperornis as in life is shown, as 

 drawn by Mr. Prentice, under my direction. Of course the 

 coloration is largely conjectural ; it is that indicated by living 

 birds of similar habits. 



