Extinct Toothed Birds of North America. 525 
admirably adapted to preserve delicate specimens, and here have been 
found the extinct Birds which form the subject of the present 
memoir. 
The geological horizon of the known Odountornithes is in the 
Middle Cretaceous, and corresponds to the strata named by the 
writer the ‘ Pteranodon beds.” The latter are included in sub- 
division number three in Meek and Hayden’s section. The accom- 
panying fossils are Mosasauroid reptiles, which are very abundant ; 
Plesiosaurs, allied to Pliosaurus, Pterodactyles, of the genus Ptera- 
nodon; and many fishes. With these occur Rudistes, and occasionally 
Ammonites, Belemnites, and various other Cretaceous invertebrates. 
The first bird fossil discovered in this region was the lower end of 
the tibia of Hesperornis, found by the writer in December, 1870, 
near the Smoky Hill River in Western Kansas. Specimens belong- 
ing to another genus of Odontornithes were discovered on the same 
expedition, The extreme cold, and danger from hostile Indians, 
rendered a careful exploration at that time impossible. 
In June of the following year, the writer again visited the same 
region, with a larger party and a stronger escort of United States 
troops, and was rewarded by the discovery of the skeleton which 
forms the type of Hesperornis regalis, Marsh. Various other remains 
of Odontornithes were secured, and have since been described by the 
writer. Although the fossils obtained during two months of explora- 
tion were important, the results of this trip did not equal onr 
expectations, owing in part to the extreme heat (110° to 120° 
Fahrenheit, in the shade), which, causing sunstroke and fever, 
weakened and discouraged guides and explorers alike. 
A considerable part of these Cretaceous deposits still remained 
unexplored, 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 Apatornis and one nearly complete skeleton of 
Hesperornis,—an ample reward for the hardship and danger incurred. 
The specimens thus secured by these various expeditions have 
since been 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 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 separate 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 
