EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



545 



evidence regarding the incisors is not positive, but 

 there were apparently two vestigial teeth, which 

 certainly could not have been functional. The 

 premolar-molar series measures 315 millimeters. The 

 internal cingula on the premolars is vestigial or en- 

 tirely wanting. The skull appears to be brachy- 

 cephalic, the estimated index being 81; the measure- 

 ment from condyles to symphysis is 645 millimeters, 

 while the width across the zygomata is estimated at 

 525 millimeters, an excess of length over breadth 

 of only 120 millimeters. The nasals are very broad 

 but at the same time short, the free length being only 

 80 millimeters. The horns are short, measuring 190 

 millimeters on the outer side, and exhibit at the base 

 the section so characteristic of this genus, which is 

 due to the flatness of the anterior or maxillary face 

 and the convexity of the posterior buttress. 



Megacerops acer Cope 



(Megaceralops acer Cope, 1873; Symborodon altirostris Cope, 

 1873; "Symborodon acer Cope," Osborn, 1902) 



Plates XVIII, CXLVI-CLII, CLVII; text figures 167, 170, 

 375, 378, 390, 392, 399, 400, 450-453, 625, 638 



[For original description and type references see p. 211] 



Type locality and geologic Tiorizon. — Cedar Creek, 

 Logan County, Colo.; Titanotherium zone, level un- 

 known. 



Specific and generic characters. — I^^, P^^. Incisors 

 greatly reduced but more persistent than in M. hucco; 

 canines small; p^ small, rounded, p^-p^ with tetarto- 

 cones distinct but smaller than in Menodus torvus, 

 a prominent mesostyle upon p^ (No. 6350), narrow 

 internal cingula; hypocones of m^-m^ large, projecting 

 on lingual side, hypocone of m' triradiate. Nasals 

 thick, short, and moderately broad (60 by 114 mm.); 

 horns c? 290 millimeters, 9 165, basal section 

 typical, summits transverse oval; cranial vertex 

 dolichocephalic; occiput high, narrow, not deeply 

 indented superiorly. Zygomata 9 with buccal proc- 

 esses flattened or slightly concave above, size mod- 

 erate, nasals to occiput 640 millimeters. This differs 

 from M. copei in the form and direction of the horns 

 and presence of a connecting crest in the male, in the 

 long, narrow parietal vertex, and in the thiclaiess of 

 the nasals. 



Materials. — This was the second species of "Sym- 

 horodon" described by Cope from Colorado in 1870. 

 The type specimen (Am. Mus. 6348) is the skull of a 

 male titanothere of medium size but with long horns, 

 from the Titanotherium zone of Cedar Creek, Logan 

 County, Colo. The exact geologic level is not known. 

 Subsequently Cope proposed the name "Symborodon" 

 altirostris for a skull of the same species (Am. Mus. 

 6350), a female, as first pointed out by Osborn in 

 1896. In addition to these two skulls there is for- 

 tunately a third (Am. Mus. 6349), probably also a 

 female, in which the right zygoma is preserved. 



Relationships to other species of Megacerops. — There 

 can be no question as to the generic relationship of this 

 animal to Megacerops hucco and M. copei. It presents 

 a number of interesting and significant points of con- 

 trast and agreement. The affinity is seen especially 

 in the form and position of the horns in the type, which 

 indicate that they were used in the manner character- 

 istic of other members of the genus. They point up- 



m^. 



B 



Figure 449.- 



-Sections and contours of skull of Megacerops 

 bucco 



A, Am. Mus. 6345a (type); horns directed upward and forward, their basal section 

 rounded with a flattened external face, the section of the upper part of the horn 

 transversely oval, nasals wide, zygomata widely expanded. B, Nat. Mus. 4705. 

 In this supposed female the horfls, as in all other members of this genus, are sub- 

 cylindrical, their basal section has a flat external face, and the remainder of the 

 section is well rounded; nasals rather short; zygomata moderately expanded. 

 One-seventh natural size. 



ward and slightly outward, and as they are strength- 

 ened posteriorly their bases do not lie very far in 

 front of the orbits. This relatively long-horned titano- 

 there was, therefore, a true Megacerops, a conclusion 

 which is further supported by the resemblance of the 

 basal horn section to that of "Menodus torvus " and by 

 its wide contrast to that of any species of Menodus, such 

 as M. trigonoceras, or of Brontotherium, such as B. gigas. 



