RESTORATION OF HESPERORNIS. 117 



The genera IcMhyornis and Apatornis, which are from the same 

 geological formation as Hesperornis, and are very fully described in the 

 following- chapters, differ widely from that genus in having biconcave 

 vertebrae, as well as in other most important characters. 



Hesperornis really stands alone among all known birds, and the 

 special characters which thus separate it will be stated in the table of 

 classification at the end of the present memoir. 



