106 Bulletin American Museum of Natural History. [Vol. XVI, 



Fig. lo. Brontot/ieriu 

 No. 42419, Sk. R.) Type. 



leidyi. (U. S. Nat. Mus. 



of exceptional importance because it is so clearly distinct 



from, its contempo- 

 raries in the Lower 

 Beds, namely, Tiia- 

 uotherium heloceras 

 and Megacerops bra- 

 chycephalus. It has 

 numerous resem- 

 blances in the shape 

 of the nasals, horns, 



canines, and back of the occiput to the great Brontotherium 



of the Upper Beds. 



MIDDLE BEDS. 



B. hypoceras Cope. — Cope's extremely fragmentary type 

 of this species (No. 6361, Amer. 

 Mus., Cope Coll.) is happily sup- 

 plemented by two skulls found 

 by Hatcher (Nos. 4702, 4273- 

 U. S. Nat. Mus.), one of which 

 is complete though crushed. 

 These prove that this animal 

 is undoubtedly a successor of 

 B. leidyi, but separated by 

 certain progressive characters, Fig. n. Brontotherinin hypoceras. 



, . . (U. S. Nat. Mus. No. 4273), cotype. 



such as the more anterior posi- 

 tion of the horns, and the entire loss of the median incisors. 



Specific Characters. — Horns of medium length, transverse oval sec- 

 tion; nasals shorter than in 5. /^id;y/; zygomata with flattened buccal 

 processes; incisors reduced to \ 



This species, although exhibiting generic resemblances, is 

 not ancestral to B. gigas. 



UPPER BEDS. 



The following species, beginning with B. gigas, form a 

 closely related progressive series which is chiefly confined to 

 the Upper Beds. One of the most distinctive characters is 



