EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



549 



(301 mm.) proves that it is a male. The incisors 

 were apparently very small and shed early, being 

 indicated solely by a single alveolus on each side. 



Figure 453. — Skull of Megacerops acer 



CJniv. Wyoming Mus. 2. Palatal view. One-sixth natural size. Shows striking 

 resemblance to the supposed female of M. bucco (Nat. Mus. 4705) and to the type of 

 Symborodon aUirosins, Affinity with Broittotlierium is also indicated in the form 

 of the canines, the highly progressive submolariform premolars, and the broad 

 molars. 



The canines are similar to Brontotherium canines 

 but of much smaller size. This degeneration of 

 the anterior teeth was shared by the first premo- 

 lar, which is a small tooth placed close to the 

 canines; in spite of its small size, however, it very 

 probably had a well-developed tetartocone as in 

 all other Brontotheriinae, indicated by the angu- 

 late postero-internal border of the much worn 

 tooth. The succeeding premolars, p^-p"*, exhibit 

 progressive development of the tetartocones (Pis. 

 CXLIII, CXLV); these cusps are still connected 

 with the deuterocones by a low crest, however, 

 and are not clearly distinct; the internal cingula 

 are feebly developed and obsolete opposite the 

 deuterocones. M^ has a heavy triradiate hypo- 

 cone. The grinding series as a whole measures 

 300 millimeters, exactly as in M. hucco, but the 

 skull of this specimen is 130 millimeters shorter 

 than in M. hucco. It would appear that in this 

 species, as in Brontotherium, the dental series does 

 not increase proportionately with the skull. 



Skull. — The lateral view of the type male skull 

 (PL CXLIII) at first sight strongly suggests that of 

 B. gigas; but a close comparison reveals the unpor- 

 tant difference that the horns are placed very much 

 farther back; they are almost directly above the 

 orbits with buttresses extending backward at the 

 base over the orbits. The horns thus obtained a 

 firm support posteriorly, a mechanical adaptation 

 that compensates for the entire deficiency of a trans- 

 verse connecting crest. When seen from in front 

 (PI. CXLIV) the horns are chiefly vertical and parallel, 

 although this is somewhat exaggerated by lateral 

 crushing; they certainly do not flare outward as in 

 B. gigas. This unique position correlated with the 

 narrowness of the occiput indicates chiefly a vertical 

 motion of the skull in using the horns and explains 

 the absence of a connecting crest, which would be of 

 great service in protecting the nasofrontal region 

 from fractures occasioned by side blows. The horns 

 differ from those of M. acer in the stronger develop- 

 ment of the transverse oval section near the summits 

 caused by the sharp development of the malar ridge. 

 Megacerops assiniboiensis Lambe 

 Text figures 205, 434, 455 



All that is known of this animal is fully stated on 



pages 239-240. The lower jaw is shown in Figure 455. 



Megacerops syceras (Cope) 



(Menodus syceras Cope, 1889) 



Text figures 186, 456 



[For original description and type references see p. 226] 



This species is known only from the coossified 

 nasals and horn cores. It is probably a member of 

 this phylum. 



Figure 454. — Restoration of Megacerops copei 

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



