TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



The transitional skull just described (Nat. Mus. 

 4258) and another skull (Nat. Mus. 4259) belong to 

 B. hrachyceplialus rather than to B. dispar, because, 

 as shown in the table of measurements above, they 

 agree far better with B. hracliycepJialus in tooth meas- 

 urements, the horn sections (figs. 414, 419) agree 

 more closely with those of B. hrachycephalus , and in 

 their tetartocone development the premolars are 

 more advanced than the type of B. hrachycephalus but 

 less advanced than in the type and referred specimens 

 of B. dispar. 



Review of primitive and progressive characters oi 

 Brontops hrachycephalus. — Nasals: All the skulls ex- 

 hibit nasals having a median terminal notch. The 

 nasals are nari'owest distally in the fourth stage of 

 growth and expand toward the extremities in the 

 seventh stage; the progressive shortening and distal 

 widening of the nasals are distinctive features of the 

 Brontops phylum. In general, abbreviation of the 

 nasals is correlated with brachycephaly. 



Horn growth: The sexual distinctions of size and 

 form in the horns are well marked. The horns not 

 only increase steadily in size in succeeding geologic 

 levels but in course of individual growth they re- 

 capitulate clearly the ancestral Eocene history of 

 this species. In young bulls the horns present an 

 elongate oval, as in the upper Eocene Protitanotherium 

 emarginatum; as the males become older the antero- 

 posterior diameter relatively decreases, the transverse 

 diameter increases, and the horn gains in height, 

 rising only 37 millimeters above the skull in the 

 fourth stage. 



Skull growth: In the meantime the summit of the 

 skull, or parietal vertex, does not perceptibly increase 

 in width; the top of the occiput extends considerably 

 back of the zygomata. The zygomatic arch sections 

 are as highly distinctive as the horn sections, since 

 the transverse diameters nearly equal the vertical 

 diameters; the zygomatic arch is relatively broad 

 and shallow and contrasts with that of Menodus 

 heloceras, which is distinctly dolichocephalic. 



Teeth; variability of incisors: There are two well- 

 developed incisors in the youngest skull transitional 

 to B. dispar (Nat. Mus. 4258); the median pair has 

 dropped out of another skull in the later part of 

 the seventh stage (Nat. Mus. 1214); but both in- 

 cisors persist in the still older type male (Nat. Mus. 

 4261). In all the male skulls the canines are promi- 

 nent and pointed, measuring from 40 to 43 milli- 

 meters in length in the males and resembling in 

 form those of B. dispar. 



Pi'emolars: The first superior premolar is small; 

 it exhibits an imperfectly formed internal crest and 

 no tetartocone. The constitution of the remaining 

 premolars, p^-p* (Nat. Mus. 4258, 1214), is mter- 

 esting as exhibiting the relative rate of evolution of 

 the internal cusps and bringing out the fact that the 

 anterior premolars are more progressive than the 



posterior; this condition is adaptively correlated with 

 the fact that the anterior premolars come into use 

 earlier than the posterior premolars, as shown by 

 Hatcher (1901.1, p. 261). Thus p^, although the 

 smallest tooth, exhibits the largest tetartocone, form- 

 ing a direct continuation of the crest of the deutero- 

 cone; p^ has its tetartocone distinctly budded off, 

 with conical sides; while in p* the tetartocone is a 

 mere cingule (figs. 406, 417). Whereas in the typical 

 B. hrachycephalus of the lower A levels the deutero- 

 cone constitutes almost the entire inner surface of 

 the crown, especially in p^"'*, and the tetartocones are 

 rudimentary, in the later geologic stages (Nat. Mus. 

 4258, 4259) the tetartocones evolve into small, low, 

 irregular-shaped cusps, leading into the condition 

 found in the type of B. validus (Nat. Mus. 4290). 

 The variability of the first superior premolar is shown, 

 as mentioned above, in one of the older B. hrachy- 

 cephalus skulls (Nat. Mus. 1214), in which it persists 

 on one side but has disappeared on the other; this 

 reduction and variability is seen also in B. dispar and 

 in Menodus. A very important character consists in 

 the fact that whereas the external cingulum on the 

 premolars is variable and feebly developed in some 

 skulls and more strongly marked in others (Am. Mus. 

 1495, Nat. Mus. 4258), the internal cingulum is con- 

 stant but never very broad, as it is in Menodus 

 trigonoceras . 



Molars: The ratio of molar length to premolar 

 length in this evolution stage is as 175 to 114. In 

 the molar series m^ also offers a very distinctive 

 character: the hypocone is a cingule, and as in B. 

 hrachycephalus an abortive crenulate crest (metaloph) 

 extends from this toward the metacone; the strong 

 cingule in front of the protocone known as the "pro- 

 tostyle" is here a feeble, inward-directed ridge. 



Brontops dispar Marsh 



{Brontops validus Marsh; " Megacerops dispar" Osborn, 1902) 

 Plates XXIV, XXXIII, LXXXVIII, XCIV; text figures 24, 



180, 375, 384-386, 389, 391, 394, 396, 399, 409-412, 419, 421, 



423, 609, 616, 719, 744 



For original description and type references see p. 223. For slceletal characters 

 see p. 664] 



Geologic horizon. — Middle beds (Chadron B) to 

 lower portion of upper beds (C). 



Specific characters. — Skull of intermediate size in 

 males; incisive border to condyles 660 to 685 milli- 

 meters, in female 610. Males brachycephalic, zygo- 

 matic index 73 to 87, average 80; zygomatic width 

 555 milhmeters (average). Females more mesati- 

 cephalic, zygomatic index 60-71, zygomatic width 

 390 millimeters. Horns progressively shifting anteri- 

 orly, more elongate, 162 to 200 millimeters. Basal 

 section roimded, summits cylindrical. Nasals pro- 

 gressively abbreviated (105 to 85 mm.), with for- 

 ward-shifting horns, spreading distally. Grinding 

 series 310 to 340 millimeters, average 329. Dental 



