100 The Badland Formations of the Black Hills Region 



Titanotherium helocerus (Cope). 



Titanotherium trigonoceras (Cope). 



Titan o therium in gens ( Marsh ) . 



Megac crops dispar (Marsh). 



Megacerops tichoceras (Scott and Osborn). 



Megacerops robustus (Marsh). 



Megacerops brachycephalus Osborn. 



Megacerops bicornutus Osborn. 



Megacerops marshi Osborn. 



Allops seraiinus Marsh. 



Allops crassicornis Marsh. 



Allops ampins (Marsh). 



Symborodon montanus (Marsh). 



Brontotherium ramosum (Osborn). 



Brontotherium dolichoceras (Scott and Osborn) 



Brontotherium platyceras (Scott and Osborn). 



Brontotherium leidyi Osborn. 



Brontotherium hatcheri Osborn. 



Symborodon copci Osborn. 



Mr. Hatcher in 1886, while searching for Titanothere re- 

 mains in South Dakota and northwestern Nebraska, discovered 

 that certain forms of the skulls of the Titanotheres are char- 

 acteristic 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 development 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 in size from the base to the sum- 

 mit of the beds was attended by a very marked development in 

 certain portions of the skeleton, noticeable among which are the 

 following: A variation in shape and an increase in the size 

 and length of the horncores as compared with the size of the 

 skulls, was attended, 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 con- 

 stant, even in the same species, shows a tendency to decrease 

 in skulls found near the summit of the beds. At the base of 

 the beds the number of incisors is from one to three on a side, 



