542 



TITANOTHERES OP ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



oblique, and strongly convex inner face; summit 

 rounded to oval in section; zygomata strongly arched, 

 buccal expansions finally extreme, in section broad 

 rather than deep; occiput moderately produced back- 

 ward behind zygomata; basisphenoidal rugosity absent, 

 vomerine septum (?) absent. 



Jaw: Jaw robust rather than deep, with shallow or 

 concave chin, coronoid strongly recurved. Angle 

 deep, rugose. 



Incisors : In most adult specimens the incisors have 

 been shed and are represented only by alveoli. Thus 

 the type of M. acer has medium-sized superior incisor 

 alveoli. A specimen (Nat. Mus. 4700) closely re- 

 sembling M. coloradensis in horn structure has per- 

 sistent superior incisors. 



Oiservations on the measurements of the Megacerops 

 series. — The members of this series are distinguished 

 from Brontotherium chiefly by the cylindrical horns and 

 by the feeble development of the connecting crest. 

 The basilar length (pmx to condyles) is shorter than in 

 any species of Brontotherium except B. hypoceras and 

 B. leidyi. The premolars as a rule are shorter and 

 smaller than in Brontotherium, as shown below. The 

 molars, however, in proportion to the basilar length 

 of the skull, are sometimes relatively larger than in 

 Brontotherium, as shown in the following tables: 



Molar index in species of Megacerops and Brontotherium 



M. acer, Am. Mus. 6350, 9 (^) 



M. acer, Univ. Wyoming (|f|) 



M. bucoo, Am. Mus. 6345a (type) (Iff) 



M. copei, Nat. Mus. 4711 (type) (H^) 



B. ?tichoceras, Nat. Mus. 8313 (Mi) 



B. platyceras, Field Mus. 12161 



B. curtum, Yale Mus. 12013 (type) 



B. gigas, Am. Mus. 492 



B. leidyi, Nat. Mus. 4249 (type) 



B. leidyi, Carnegie Mus. 93 



Anteroposterior and transverse dimensions of p* and w? in species 

 of Megacerops and Brontotherium, in millimeters 



M. acer?, Am. Mus. 6350 (type of M. 

 altirostris) 



M. acer, Univ. Wyoming , 



M. bucco. Am. Mus. 6345a (type) 



M. bucco, Am. Mus. 6353 



M. bucco, Nat. Mus. (skull A) 



B. gigas hatcheri, Nat. Mus. 4262 



B. gigas, Am. Mus. 492 



B. gigas (hatcheri), Carnegie Mus. 341 



83 

 80 



■74 



99 



84 



o Estimated. 



The internal cingula of the premolars are usually 

 more reduced than in Brontotherium. 



The close kinship of Megacerops to Brontotherium 

 is revealed in many details of the incisors, canines, 

 premolars, and molars, in the great expansion of the 

 zygomata, and in the possession of a midparietal 

 protuberance. 



Measurements of species of Megacerops and Brontotherium, in 

 millimeters 



Standard measurements in the Megacerops phylum, in millimeters 



