EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TXTANOTHERSE 



541 



Our knowledge thus rests chiefly on six skulls and 

 two lower jaws as representing this genus. There 

 are also isolated horns and the top of a cranium in 

 the American Museum collection and fragmentary 

 skeletal material (figs. 625, 629, 638). In the IJni- 

 versity of Wyoming there is an excellent skull of 

 Megacerops acer. 



General characters. — These specimens agree in the 

 exceptionally small size of the canines, in both the 

 males and the females. The nasals are thin in two of 

 the species, M. hucco and M. copei. M. acer is some- 

 what divergent in the thickening of the nasals and 

 in other characters. 



It is thus apparent that Megacerops is a separate 

 collateral phylum, resembling Brontotherium in the 

 elongation of the horns and paralleling Menodus in 

 the degeneration of the incisors; but it differs from 

 both phyla in the shape and position of the horns 

 and in the approximation of the canines toward the 

 median line. 



STceleton. — The skeleton is known only from a few 

 portions associated with M. acer in the American 

 Museum, which indicate that the animals of that 

 species were rather small. 



While generally of smaller size and differing widely 

 from Brontotherium in the entire absence or vestigial 

 character of the incisor teeth, the position and basal 

 section of the horns, and some other characters, these 

 animals present many resemblances to Brontotherium, 

 especially in the elongate horns — in some cases oval 

 in top section — in the backward prolongation of the 

 occiput, the broad contact between the postglenoid 

 and post-tympanic, the roundness of the malar bones, 

 the suddenly projecting buccal expansions of the 

 zygomatic arches, bluntness of the canines, round- 

 ness and bluntness of the internal cusps of the pre- 

 molars; also the abbreviation of the premolar series 

 and reduction of the cingula, the strongly recurved 

 coronoid, and the general contour of the jaw. 



The common characters of these Megacerops species 

 are clearly perceived in a comparison of the transverse 

 sections of the horns and of the nasals, which are 

 highly characteristic. The main features of corre- 

 spondence between Megacerops copei and Menodus 

 torvus are, first, that the nasals are very thin, even 

 in the portion between the horns; second, that the 

 horns are set widely apart at the base. (This char- 

 acter is obscured by lateral crushing in the male 

 type skull of M. copei.) Reference to the detailed 

 descriptions of the skulls in these two species 

 shows that they are closely related but that M. 

 copei is more primitive, especially in the retarded con- 

 dition of the tetartocones, the section of the buccal 

 processes, and the persistence of the reduced in- 

 cisors. 



SYSTEMATIC DESCRIPTIONS OF GENERA AND SPECIES 

 IN THE MEGACEROPS PHYLUM 



Megacerops Leidy, 1870 



(Megaceratops Cope, 1873, in part (M. acer); Symborodon Cope, 

 1873, in part (5. hucco, S. altirostris) ; "Symborodon" Osborn, 

 1902) 



Plates XVIII, CXLIII-CLX, CLXXXVI; text figures 24, 164, 

 167, 169, 375, 378, 390, 392-394, 398-400, 434, 448-456, 625, 

 629, 638, 640, 719, 744, 746 



[For original description and type references see p. 208. For skeletal characters 

 see p. 691] 



Generic characters.— Incisors, typically vestigial. Ca- 

 nines small, obtuse. Grinding teeth without cingula, 

 deflected upward. Premolars with progressive tetar- 

 tocones. Skull brachy cephalic to hyperbrachyce- 

 phalic (zygomatic index 84), brachyopic. Nasals 

 slender, narrow, decurved, abbreviated progressively. 

 Horns set vertically, typically without connecting 

 crest; placed above orbits; rounded in section. 



The genotype of Megacerops (1870) is the species 

 M. coloradensis Leidy, represented by nasals and 

 horns (fig. 448). The genotype of "Symborodon" 

 (1873) is the species S. torvus Cope, which is repre- 

 sented by a lower jaw. (See p. 211.) This jaw does 

 not belong to the same phylum as Megacerops because 

 it proves to be that of a Menodus. 



General characters. — Dentition: I^°. Incisors re- 

 duced, vestigial, or wanting; canines reduced, obtuse; 

 opposite grinding series arched to strongly arched; 

 upward flexure of face and premolar series as seen in 

 side view extreme; length of premolar-molar series 

 generally less than two-fifths that of the skull, from 

 premaxillary tips to occipital condyles; premolar series 

 very short; internal cusps of grinding teeth low, robust, 

 well rounded, ectolophs sharply depressed to the 

 crowns of the teeth; anteroposterior diameter of m- 

 and m' less than transverse diameter; no cingula be- 

 tween grinders; P||; p' subc[uadrangular, outer wall 

 not overlapped posteriorly by ectoloph of p^; premolar 

 tetartocones exhibiting early and pronounced develop- 

 ment; premolars with internal cingula blunt, reduced 

 or absent, external cingula variable; molars without 

 internal cingula, external cingula faint or absent; 

 hypocone of m' prominent, triradiate. 



Skull: Skull proportions mesaticephalic to brachy- 

 cephalic; facial portion of skull much abbreviated; 

 premaxillaries contracted; cranial portion of skull 

 elongate; anterior narial aperture high and narrow; 

 preorbital malar bridge very narrow, mainly com- 

 posed of the median ridge, which is very prominent, 

 subcylindrical, in side view concealing the infraorbital 

 foramen almost entirely; anterior portion of malar 

 stout, rounded; malar below postorbital process 

 strongly convex; free nasals tapering, progressively 

 abbreviated; horns of medium to large size, forward 

 shifting slight or wanting, basal section deep antero- 

 posteriorly, with antero-external or maxillary face flat. 



