CAUSES OF THE EVOLUTION AND EXTINCTION OF THE TITANOTHERES 



831 



Brontops. — The elongation of the midcranium 

 having been effected, Brontops began to develop 

 buccal brachycephaly and finally carried it to an 

 extreme in B. rolustus and "Diploclonus" amplus. 



Allops. — Allops lengthened the skull, and especially 

 the molar series, but also showed some brachycephalic 



gressively increase rather than decrease in size; also 

 the close alliance of Menodus and Brontops points 

 rather to the possible derivation of Menodus from 

 some subdolichocephalic form of Manteoceras, like 

 M. uintensis, yet retaining the sharper-edged canines 

 of certain species of Bridger Manteoceras. At any 



£btitanops V^^ J E 



A^anteocera-s 

 Figure 745. — Skulls of Eocene titanotheres, showing changes in proportion and in development of horns 

 Hornless series: A, EotUanops borealis: B, Limnohyops prisons: C, Palaeosyops leiiyi; D, Tdmatherium ultimum. 

 Precociously horned series: E, Manteoceras manteoceras: F, Mesatirhinus petersoni: Q, Metarltinus earlei: H, DolichorUnus hyognathus. 



tendency. Its transitional character between Bron- 

 tops and Menodus points to the convergence of those 

 two lines at a not very distant date, perhaps during 

 some stage of the upper Eocene. 



Menodus. — Menodus was probably not derived from 

 Telmatherium, for the incisors of that genus pro- 



rate, Menodus carried dolichocephaly to an extreme, 

 yet shows a slow progressive broadening across the 

 zygomata {M. heloceras to M. giganteus). 



Mesatirhinus. — The skull of Mesatirhinus as com- 

 pared with that of Eotifanops had already lengthened 

 all the parts. This dolichocephalic tendency became 



