492 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



The canine is stout and conical; the incisors are sub- 

 spherical; the external cingula are not as sharply 

 defined as in Menodus. 



Female skulls: The females of this species (Nat. 

 Mus. 4738) are, according to the principle expressed 

 above, distinguished by smaller and much less robust 

 canines; the greatly reduced canines, the more slender 

 zygomatic arches, and the inferior dimensions of this 

 skull and teeth as a whole constitute the distinctive 

 sexual characters. The internal and external premolar 

 cingula are quite as strong as in the male representa- 

 tives of this species. 



The skull forming Nat. Mus. 4738 (fig. 391, A) 

 presents rather slender zygomatic arches; the nasals, 

 although imperfectly preserved, indicate a slender, 

 unexpanded contour. This skull is in the eighth stage 

 of growth, yet it retains two superior incisors on each 



Figure 420. — Restoration of Brontops rohustus 

 By Charles R. Knight. Ahout one-ninth natural size. 



side. The canines measure only 35 millimeters an- 

 teriorly as compared with 47 in the largest males. The 

 protocones of m^, m' are rather low, are set well in 

 from the inner margin of the tooth, and have a slight 

 concavity on the lingual side near the top — an individ- 

 ual variation. A peculiar feature of this specimen is 

 the reduplication of the tetartocone of p"*, an anomalous 

 character (see explanation above) also observed in B. 

 hracTiycepJialus (Nat. Mus. 4259) and in the type of 

 Alloys crassicornis as well as in the type of Menodus 

 (Menops) varians. A character reversional to B. 

 hracTiycephalus is the abortive crenulate metaloph of 

 m', extending forward and inward from the hypocone. 

 In palatal view the less robust character of the basi- 

 sphenoid and the narrowness of the zygomata are well 

 illustrated. Another character, which may be an 

 individual variation, is the nonclosure of several of the 



sutures, the median suture between the nasals and 

 frontals being still widely opened. Similarly the 

 external portion of the mastoid is still well defined 

 between the post-tympanic and the exoccipital proc- 

 esses. The post-tympanic and postglenoid processes 

 of the squamosal are not conjoined below. 



Extension oj B. dispar into upper beds, or C. — ■ 

 Although B. dispar is especially characteristic of the 

 middle Titanotherium zone, evidence of its occurrence 

 in the upper beds even to a point a little below middle 

 C is apparently indicated by skull No. 1217 in the 

 National Museum. This fine specimen with the lower 

 jaw attached (PI. XCIII) appears to represent a very 

 large male of B. dispar, the length from condyles to 

 premaxillaries being estimated at 702+ millimeters, 

 m^-m^ 196, mi-mj 230. The detailed measurements 

 of the grinding teeth, however, as in the large upper 

 zone brontotheres, are smaller through- 

 out than those of the large B. dispar, 

 Nat. Mus. 4696, from the middle beds. 



Brontops robustus Marsh 



{" Megacerops robustus" Osborn, 1902) 



Plates XVIII, XX, XXXIII-XXXVI, XLIII, 

 XLVII, XCV-CIII, CXCV-CCXXIX; text 

 figures 24, 179, 387, 389, 393, 394, 396, 399, 

 400, 409-412, 420-423, 482, 483, 510, 608, 

 618, 622, 624, 626, 628, 630, 635, 636, 639, 

 640, 650, 651, 653-655, 657-660, 669, 685, 

 686, 693, 699, 712, 719, 727, 744 



[For original description and type references see p. 222. For 

 skeletal characters see p. 666] 



Geologic horizon. — Lower level of the 

 upper Titanotherium zone (C). 



Specific characters. — Length of skull 

 765 to 813 millimeters, strongly brachy- 

 cephalic, zygomatic index, 77 to 87, 

 average 79; backward prolongation of 

 occiput slight or absent; horns shifted 

 forward and inclined forward, basal sec- 

 tion broadly transverse oval, summit 

 obtuse, outside length 207 millimeters (type); nasals 

 abbreviate, free length 76 millimeters, breadth 149, 

 nasal index 51 ; dental index 46, same as in B. brachy- 

 cephalus and B. dispar, total grinding series 345 

 to 376 milhmeters; molars 215 to 236, average 

 223; canines rounded in section, obtuse, stout, length 

 of crown 42 to 48 millimeters; tetartocones of 

 p^"^ somewhat less developed than in typical B. 

 dispar. 



This species marks the end of the Brontops phylum, 

 toward the end of lower Oligocene time. It is related 

 to B. brachycephalus but does not directly succeed 

 any of the known mutations of B. dispar. 



General characters. — This is the most fully known 

 species of titanothere. The fine type skull and skele- 

 ton in the Yale Museum (Yale Mus. 12048) represent 

 a middle-aged animal. Both were found in "Corral 



