EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



447 



of the vertex to the deeply concave contour, corre- 

 lated with the development of the enormous lateral 

 pillars and supraoccipital rugosities to which are 



Figure 378. — Occipital view of skulls in 

 different phyla of Oligocene titanotheres, 

 showing widening of the occiput and 

 development of its lateral pillars and 

 superior crests in the final stages 



After Osboni. A, Menodus Jieloceras, Am. Mus. 6360 

 (type); Chadron A. B, Megacerops acer, Am. Mus. 

 6348 (type): ?Chadron C. C, Allops marski, Harvard 

 Mus.; ?Chadron B. D, Menodus trigonoceras, Am. 

 Mus. 1067,9; f^hadron C. E, Megacerops bucco, Am. 

 Mus. 6345a,d' (type); Chadron C. F, Brontotherium 

 gigas, Am. Mus. 492, .J ; Chadron C. 



attached the powerful muscles and tendons of the 

 neck. Figure 378 illustrates these extremes of struc- 



ture. In general the massiveness of the occiput is 

 directly correlated with the size, location, and function 

 of the horns. Thus in Megacerops acer (fig. 378, B) 



poc. 



.p.pe?: 



p.oc. 



B 



» ^ft ~ TTLp.per. 



Figure 379. — Influence of progressive brachy- 

 cephaly on the auditory region of perissodactyls 



A, Dolichocephalic, Eguus caballus; B, mesaticephalic, Tapirus; 

 C, mesaticephalic, DiceroTMnus sumatrensis; D, brachycephalic, 

 Rhinoceros sondaicus. Note disappearance of mastoid portion 

 of periotic (m. p. per.) and inclosure of auditory meatus (e. a.m.) 

 inferiorly. Parallel changes occur in the titanotheres. 



the occiput is narrow, slender, and slightly indented, 

 in keeping with the relatively slender horns, and 

 presents a very wide contrast to the broad, rugose. 



