112 THE WHITE RIVER BADLANDS 



are characteristic of certain horizons in the beds, and this 

 indicated to him the importance of keeping an exact record 

 of the horizon from which each skull or skeleton was taken. 

 Continued search showed that a regular and systematic de- 

 velopment took place in these animals from the base to the 

 top of the beds. The most notable change was a gradual and 

 pronounced increase in size. Hatcher says : "This increase 



Figure 51 — Skulls of Tit another -ium elatum. Upper skull, male; lower 

 skull, female. Osborn, 1896. 



in size from the base to the summit of the beds was attended 

 by a very marked development in certain portions of the 

 skeleton, noticeable among which are the following : A varia- 

 tion in shape and an increase in the size and length of the 

 horncores as compared with the size of the skulls was at- 

 tended, near the summit of the beds at least, by a decided 

 shortening of the nasals. There were also changes taking 

 place in the dentition of these animals, especially in the 

 number of incisors and in the structure of the last, upper, 

 true molar. The number of incisors, though probably never 

 constant, even in the same species, shows a tendency to 

 decrease in skulls found near the summit of the beds. 



