INTRODUCTION. 3 



A considerable part of these Cretaceous deposits still remained unex- 

 plored, and in the autumn of 1872, a third expedition through this territory 

 was undertaken by the writer, with a small party. Additional specimens 

 of much interest were secured, including the type of the genus Apatomis, 

 and one nearly complete skeleton of Hesperornis, — an ample reward for the 

 hardship and dang-er we incurred. 



The specimens thus secured by these various expeditions have since 

 bee'n supplemented by important additions, collected in the same general 

 region by different parties equipped and sent out by the writer, who no 

 longer could give his personal supervision to work in that field. The fossil 

 Birds procured in this region since 1870, by these different expeditions, 

 include remains of more than one hundred different individuals of the 

 Odontornithes. These are all in the Museum of Yale College, and form 

 the material on which the present volume is based. 



A study of this extensive series of Bird remains brings to light the 

 existence in this class of two widely separated types, which lived together 

 during the Cretaceous period, in the same region, and yet differed more 

 from each other than do any two recent birds. Both of these types 

 possessed teeth, a character hitherto unknown in the class of Birds, and 

 hence they have been placed by the writer in a separate sub-class, the 

 Odontornithes. One of these groups includes very large swimming- birds, 

 without wings, and with the teeth in grooves (OdontolcaJ, and is represented 

 by the genus Sesperomis. The other contains small birds, endowed with 

 great powers of flight, and having teeth in sockets (Odontotormce), and 

 biconcave vertebrae ; a type best illustrated by the genus IcMhyomis. 

 Other characters, scarcely less important, appear in each group, and we 

 have thus a vivid picture of two primitive forms of bird structure, as 

 unexpected as they are suggestive. A comparison of these two forms 

 with each other, and with some recent birds, promises to clear away many 

 difficulties in the genealogy of this class, now a closed type ; and hence 

 they are well worthy of the detailed description and full illustration here 

 devoted to them. 



