g6 BuUetifi American Museum of Natural History. [Vol. XVI, 



arises from the imperfect condition of the type. With this 

 species, however, is probably associated a skull (Nat. Mus, 

 No. 4260) definitely recorded by Hatcher as from the Lower 

 Beds and exhibiting a number of undoubted Titatwtherimn 

 characters. 



MIDDLE BEDS. 



T. trigonoceras Cope. — Animals of middle size with long 

 metapodials, represented by numerous specimens in the 

 American and National Museums, show the state of evolution 

 of this phylum in the Middle Beds. As compared with T. 

 heloceras there is a marked progression in size, in the shaping 

 of the horns, the development of the paired premolar cones, 

 and of the cingulum on the grinding teeth. 



UPPER BEDS. 



T. ingens Marsh (S}^., Menops varians Marsh). — These 

 very large, long-skulled and long-footed Titanotheres, first 

 described by Marsh as Brontotherium ^ ingens, are definitely 

 recorded by Hatcher from the Upper Beds, and show a cor- 

 responding increase in size and in all other progressive char- 

 acters. The incisors are vestigial, the canines very long and 

 powerful; the cingulum on the grinding teeth is very pro- 

 nounced in both males and females. There is a marked 

 difference between the sexes in the ' size of the horns and 

 canines. 



Phylum II. 



Brachyce phalic Titanotheres with short horns shifting for- 

 wards, rounded to oval at summits; nasals progressively reduced 

 in length and broadening at the extremities ; one or two pairs of 

 persistent incisor teeth, above and below; canines pointed, of 

 medium length; premolars, ^^; zygomata progressively expand- 

 ing into convex buccal processes; cranium not greatly produced 

 behind zygomata. 



^ As explained below, the name Brontotherium applies to the animals called Titanops 

 by Marsh. 



