486 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



istic features of this species; the horns are somewhat 

 smaller than in ProtitanotJierium emarginatum of the 

 upper Eocene. They present an elongate oval section, 

 with sloping anterior face. The zygomata are fairly 

 developed and typical in section. The anteroposterior 

 and transverse diameters of the molars are subequal. 

 The tetartocone of p* is a concave spur from the 



the angle projects downward and backward; the 

 coronoid is high; the molars are relatively larger. 



As arranged in the order of sequence from the base 

 to the summit of the lower zone the remarkable series 

 of skulls in the National Museum (see table on p. 480) 

 gives us a complete series of ascending mutations toward 

 the higher stages of B. dispar in the middle beds. 



Proportions of teeth of Brontops brachycephalus, 

 in millimeters 



Figure 418. — Skull (B) and horn region (A) of Brontops brachycephalus? 

 Nat. Mus. 1214; Chadron A 3. The horns are more progressive than in the type of this species and in their 

 trihedral section suggest Allops marsM, The dental measurements agree better with those of B, bra- 

 chycephalus, although the premolar series is unusually long. The large canines probably indicate male 

 sex. One-third natural size. 



cingulum. P' is set on a line with p^ (outer wall) and 

 is separated by a diastema from the canine. The 

 canines are procumbent, as in Nat. Mus. 4261, with 

 faint external cingula. M^ has crenulate metalophs, 

 m3 a hypoconulid not greatly different from that of 

 Menodus. The jaw of this specimen is shown in Fig- 

 ure 396, A. As compared with Teleodus avus this 

 jaw has the horizontal ramus very shallow anteriorly; 



Transitional stages from Brontops hra- 

 cJiycephalus to B. dispar. — A transitional 

 stage is represented by a rather primitive 

 skull (Nat. Mus. 4940), from the top level 

 (A 3) of the lower beds, which is interme- 

 diate in the length of its horns between 

 the type of B. brachycephalus and some 

 of the more primitive varieties of B. 

 dispar. This skull taken as a whole is 

 also somewhat more progressive in char- 

 acter than the male type and female par- 

 atype above described. The nasals are 

 thin, elongate, and contract anteriorly. 

 The horns present a decidedly oblique 

 oval form at the base but round up 

 toward the summit into a more cylin- 

 drical section; they rise 60 millimeters 

 above the midline of the skull, and since 

 this animal is well advanced toward the 

 eighth stage of growth, this is probably 

 the maximum development of the horns 

 in the males at this stage. The pos- 

 terior nares open somewhat more ante- 

 riorly than in the type of B. brachy- 

 cephalus. 



There is but one incisor above on each 

 side and the first superior premolar is rep- 

 resented by its alveolus only on the right side, the alve- 

 olus being closed on the left. It is important to note 

 this reduction of the median incisor, or ii, and the shed- 

 ding of the first superior premolar. The remaining pre- 

 molars, p^-p*, show a slightly stronger development of 

 the tetartocones than in the B. brachycephalus type and 

 paratype, although in p* the deuterocone still constitutes 

 the main internal portion of the crown, as in the type 



