EVOLUTION OP THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



487 



of B. hrachycephalus ; in m^ the abortive metaloph ex- 

 tends outward from tlie cingule without a distinct 

 hypocone. 



A second transitional skull from upper A (A 3) (Nat. 

 Mus. 1214; fig. 418) may also be considered transi- 

 tional between B. iracJiycepJialus and B. dispar. It 

 is in the seventh growth stage. The median pair of 

 incisors (i|) has also dropped out entirely; the first 

 superior premolar persists on one side and has dis- 

 appeared on the other, so that the formula in both 



somewhat similar to that in Leidy's type of Mega- 

 cerops; the horns show a more trihedral section at the 

 base than in the preceding specimens owing to the 

 incipient development of the connecting ridge (cr). 

 A transitional stage from lower B is represented by 

 the young male skull (Nat. Mus. 4258) found in B, 

 or the middle levels — namely, 71.45 feet above the 

 Cretaceous Pierre shale. It gives us valuable details 

 of the architecture of the antei'ior portion of the face. 

 Seen from above (Pis. LXXXIV, LXXXV) the 



A 



D 



Figure 419. — Sections and contours of skulls of Brontops hrachycephalus (A) and B. dispar (B, C, D) 



, Nat. Mus. 4259; B, Nat. Mus. 4941; C, Nat. Mus. 4703; D, Nat. Mus. 4290 (type of B. validus). All one-eighth natural size. The more progressive members of B. 

 brachycephalus (A) approach B. dispar but are distinguished by their smaller size, small horns, low connecting crest, and relatively slender zygomata. In B. dispar 

 (B, C, D), which is characteristic of Chadron B, the horns are much stouter with rounded basal section, the nasals thicker and shorter, and the zygomata more 

 massive. The more progressive members (D) approach B. roiustus. Note the sunilarity of the buccal section in D to that of Am. Mus. 518, 9 (flg. 424; see also 

 fig. 376, showing position of sections and contours). 



specimens is I', P*~', demonstrating absolutely the 

 variability of p^ The marked primitive feature of 

 the premolars is that while in p^, p^ the tetartocones 

 are becoming more distinct and rounded, in p* the 

 deuterocone still forms almost the entire inner surface 

 of the crown. In this, as in the preceding types, the 

 metaloph extends from the hypocone cingule of m' 

 outward toward the metacone. The rugosity on the 

 basisphenoid is very prominent; the nasals are of 

 medium length in this specimen, expanding slightly at 

 the sides, which is an age character, and their form is 

 101959— 28— VOL 1 34 



frontals cap the nasals and constitute the entire tips 

 of the horns. In side view, however (PI. LXXXIV), 

 the nasals are seen to support the main anterior por- 

 tion of the base of the horn, resting postero-inferiorly 

 on the lacrimals, elements which are not quite so ex- 

 tensive as represented in Plate LXXXIV, A^. Sub- 

 sequently the lacrimal coalesces with the malar, and 

 we speak of the ridge extending down from the side of 

 the horns as the malar or external ridge. In palatal 

 view the posterior nares open between m^ and m'. 

 The vomer (PI. LXXXVI) extends well backward. 



