EVOLUTION OF THE SKULL AND DENTITION OP OLIGOCENE TITANOTHERES 



579 



Materials. — This grand stage, the "flat-horned" 

 brontothere, which represents the climax of the 

 evolution of the long-horned titanotheres, is compara- 

 tively rare. It is represented by the type horns in 

 the Harvard Museum, also by another pair of horns 

 in that museum. The well-preserved skull of an 

 old male in the American Museum (No. 1448) was 

 discovered by one of the expeditions under Mr. 

 J. B. Hatcher; it agrees closely with the type and may 

 be taken as a neotype. The finest skull known is 



backward. The nasals are now mere pendent tuber- 

 osities from the base of this plate and the external 

 nares open directly forward. At the sides of the head 

 there were the great cheek processes. As this was the 

 face of an animal in the seventh stage of growth, we 

 may imagine the grotesque appearance of an old bull 

 in the tenth stage. The breadth of the skull has now 

 reached its extreme, the width of the zygomatic 

 arches being about 32 inches (815 mm.), exceeding 

 the length (distance from tips of the premaxillaries 

 to the occipital condyles) by 3.35 inches (85 mm.). 



It is evident when we compare these correlated pro- 

 gressive characters of the skull with the compara- 

 tively stationary characters of the teeth that the chief 

 force of evolution or of selection was directed toward 

 the development of the horns and buccal processes; 

 while the teeth, so essential to the vitality of this 



Figure 480. — -Restoration of Brontotherium plalyceras 

 By Charles E. Knight. About one-ninth natural size. 



that in the Field Museum (No. 12161), discovered 

 by an expedition under Mr. E. S. Riggs. 



Brontotherium platyceras is one of the most re- 

 markable and exceptional imgulates known to science. 

 Both in front and in side view the head presents a 

 unique and extraordinary appearance. 



The connecting crest between the horns has grown 

 to such a height and the horns to such a breadth that 

 in front view the head terminates in a great vertical 

 plate 6.75 inches deep and more than 18 inches broad 

 at the widest part. From the sides of this plate the 

 horns extended upward and were gently recurved 



species, were stationary. It is difficult to account for 

 the sudden extinction of this apparently prosperous 

 and numerous race of brontotheres; but the mechanical 

 imperfection of the teeth may have been one of the 

 factors in the extinction as fully discussed in a sub- 

 sequent chapter. 



Teeth. — An exception to the general retrogression 

 of the teeth is the persistence of the two upper in- 

 cisors, as in all previously described males of the 

 Brontotherium phylum. As preserved in the neotype 

 skull (Am. Mus. 1448) the summits of the superior 

 incisors are smooth and oval, with the long axis 



