EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



485 



overhanging the sides of the maxillae; the horn sec- 

 tions are obliquely placed ovals that converge ante- 

 riorly. 



Figure 416. — Reconstruction of crushed skull of 

 Brontops brachycephalus 



Am. Mus. 1495; Chadron A; South Dakota. Ai, Skull fully 

 adult, less brachycephalio than the type of B. brachycephalits, 

 and one of the most primitive known of all Brontopinae. One- 

 sixth natural size. Very small horns (fundamentally similar 

 to those of the Eocene Manteoceras) consisting of swellings on 

 the frontals which overlap the nasals. The surface of the horn 

 swelling (Ai) is roughened and pitted as if for the attachment 

 of a tough or even horny epidermal cap. A2, Horn region. 

 One-third natural size. The upper teeth of this skull are shown 

 in Figure 417; the lower jaw in Figure 413, B. 



The cranium proper is narrow, measuring 

 80 millimeters across the vertex; the occiput 

 is broken away; the zygomata arch outward 

 and give a brachycephalic character to the 

 skull, although the measurements show that 

 it is in reality mesaticephalic, the zygomatic 

 index being only 64. This mesaticephaly is a female 

 character. In section these arches are deep, thin 

 bars. The posterior nares are placed very far back, 

 opposite the posterior face of m^ The vomer ex- 

 tends well back on the basisphenoid with a median 



keel in the portion lying between the pterygoids, 

 extending back almost as far as the basisphenoid 

 rugosity, as seen also in succeeding types. The ali- 

 sphenoid canal, foramen ovale, f. lacerum medium, 

 and f. condylare are clearly defined. The somewhat 

 rugose junction of the basisphenoid and basioccipitals 

 foreshadows the prominent projection for the rectus 

 capitis muscle, which is so characteristic of succeeding 

 species of Brontops. 



Dentition. — Of the incisors two (i\ i^) persist on one 

 side and only one on the other. The premolars are 

 completely worn down, but so far as one can observe 

 the deuterocone constitutes almost the entire inner face 

 of the crown "of p*. The hypocone of m' is a cingule 

 connected with a very interesting abortive metaloph, as 

 seen in some other titanotheres. The inner contour of 

 the crown is very rounded, the primitive triangle being 

 scarcely altered by the addition of the slight protostyle 

 and hypocone. 



Type male skull. — The upper surface of this aged 

 male skull (Nat. Mus. 4261, PI. LXXXVII, figs. 399, 

 410, 411, 414) is especially well preserved. Although 

 compared with the female it is a relatively large and 

 adult animal, in the ninth stage of growth, the horns 

 rise only 85 millimeters above the nasals (outside meas- 

 urement). They exhibit the primitive or original 

 position of these structures — namely, very far back and 

 immediately above the orbits, directed outward. The 

 very robust section of the horn base is a male character 

 distinguishing this from the above-described female 

 horn. This skull is very interesting, moreover, because 

 it illustrates the great breadth (480 mm., estimated) 

 attained by the zygomatic arches in this low geologic 

 level. This brachycephaly is a male character. The 

 dental formula is I^~, P*~'. The premolars are dis- 

 tinguished by narrow internal cingula and rudimentary 

 tetartocones; the teeth exhibit faint external and de- 

 cided internal cingula, but the most decided feature is 

 the sessile character of the tetartocones, which occupy 

 barely one-third of the inner face of the premolar 



Figure 417. — Left upper grinding teeth of Brontops brachycephalus 



Am. Mus. 1495. One-third natural size. These teeth agree well in form and measurements with 

 those typical of B. brachycephalus. They difler from those of Allops walcotii (type) in minor 

 points, and from those of Menodus proutii in being less elongate anteroposteriorly, especially 

 the molars, which also have the internal eingulum less pronounced. The premolar pattern 

 resembles that of aU the more primitive members of the menodontine group. (See flgs. 413, 416.) 



crowns. The incisors are absolutely transverse in 

 position with perfectly smooth, rounded crowns; the 

 canines are quite large. 



The American Museum specimen (No. 1495), the 

 skull of a yoimg animal, also exhibits the character- 



