EVOLUTION OP THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



547 



A very distinctive feature is the fact that it is not 

 incurved in the median line superiorly. 



The buccal expansion of the zygoma is preserved 

 only in one of the female skulls (Am. Mus. 6349), in 

 which it is seen to be broad and flattened above, as 

 in M. hucco, paralleling that of Brontotherium, gigas. 

 The portion of the malar preserved in No. 6350 is 

 stout and rounded. 



Teeth. — That the superior incisors were greatly 

 reduced is demonstrated by the small alveoli in the 

 female skull Am. Mus. 6350, which, however, are 

 not so small as in Menodus. The rounded first pre- 

 molar resembles that of M. copei; the slightly defined 

 cingulum on the inner side of the premolars and the 

 rather retarded development of the tetartocones dis- 

 tinguish these teeth from those of M. hucco. There 

 is a well-marked mesostyle on p''. As in M. hucco 

 and M. copei, the distance from the anterior edge of 

 m' to the premaxillary border and the proportionate 

 length of the premolar series arc excessively short, com- 

 pared with Menodus, Allops and Brontops, and even 

 compared with Brontotherium (Pis. CLI, CLII, and 

 CLXV). The upward flexure of the premolar series 

 and elevation of the canines and incisors is very 

 marked and in connection with the elevated position 

 of the nasals and perpendicularity of the horns must 

 have imparted a peculiar retrousse character to the 

 face (PI. CXLVI). 



Sexual characters. — The males are well distinguished 

 from the females by the length of the horns, which is 

 290 millimeters in the male (Am. Mus. 6348) as com- 

 pared with 177 in one female (Am. Mus. 6350) and 

 138 in the second female (Am. Mus. 6349). The basal 

 section of the horns is substantially similar, and the 

 summits are transversely oval in section; but in one 

 of the female skulls (No. 6350) the horns project 

 forward without recurvature, while in the other they 

 are recurved as in the male skull. There is a strong 

 convexity of the cranial vertex in the frontoparietal 

 region (a family character) in the male No. 6348 and 

 the female No. 6350, which is feebly developed in 

 the female No. 6349. The horn of the latter has a 

 malar ridge. 



Additional ohservations on skulls of Megacerops acer. — 

 The type of M. acer is a relatively long and slender 

 male skull (Am. Mus. 6348) lacking all the teeth, 

 the zygomatic arches, the maxUlaries and premaxil- 

 laries. The skull is sharply distinguished from an}^ 

 previously described by its obtuse and short but 

 tapering nasals, long, recurved horns with deep an- 

 teroposterior basal section and oval tips, a narrow 

 vertex, and narrow, simple occiput. In addition to 

 these characters there appears to be a strong convexity 

 in the central portion of the top of the cranium not 

 altogether due to crushing, which is characteristic of 

 Brontotheidum and Megacerops. The basal section of 

 the horns indicates that they are strengthened not only 

 by a connecting crest but by a posterior ridge passing 



backward above the orbits, which is separated by a 

 flat surface from the malar ridge. 



The skull Am. Mus. 6350 resembles that of M. 

 acer in the abbreviation of the nasals and narrowness 

 of the vertex and the occiput and differs from M. 

 acer in characters which Cope thought to be specific 

 but which are now found to be sexual. These female 

 characteristics are the relatively short horns and com- 

 paratively low connecting crest, as shown in the section 

 (fig. 450); the only difference of note is the convexity 

 between the malar and posterior ridges of the horns. 



The anterior teeth of this type have been broken 

 away and lost since the original description. The 

 superior incisors are represented by two small alveoli, 

 placed upon a nari'ow border, indicating that these 

 teeth were small and disappeared in old age. The 

 formula was, therefore, I^"^, P-^. The crowns of the 

 canines have been destroyed. These teeth were of 

 small size, apparently as in M. hucco. The first pre- 

 molar is a small tooth with three cusps (protocone, 

 deuterocone, and tritocone); the second, third, and 

 fourth premolars e.xhibit four well-developed and dis- 



FiGURE 451. — Upper part of occiput of 

 Megacerops acer 



Am. Mus. 6351, showing pits tor the ligamentum nuchae 

 and rugosities for the recti capitis laterales. One-fourth 

 natural size. 



tinct cusps, including a convex tetartocone. A fea- 

 ture which may be specifically characteristic is the 

 distinct mesostyle upon p*. A crenulate and not 

 sharply defined cingulum is observed on the inner 

 side but is entirely wanting on the outer side of the 

 premolars. This more or less vestigial condition of 

 the cingulum is also a distinctive character of the 

 series to which this animal belongs, as well as of the 

 old males of the species of Brontotherium of the upper 

 beds. The molars are very broad. On m' the hypo- 

 cone is triradiate, well developed, but not distinct 

 from the cingulum. Another well-marked feature is 

 the prominence and roundly blunt character of the 

 hypocones of m'-m'. 



A female skull (Am. Mus. 6349) fortunately pre- 

 serves the right zygomatic arch, indicating that this 

 is broad and somewhat flat superiorly, having a sec- 

 tion similar to that of Brontotherium gigas but less 

 robust. A distinctive character is the breadth of the 

 postglenoid process. The occiput has the relatively 

 slender proportions seen in the type and cotype. The 

 rugosities on top of the occipital pillars are much less 

 extensive than in Megacerops rohustus. 



