EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



491 



uniform and constant, as shown in the table above. 

 In the males the superior premolar-molar series 

 measures 310 to 345 millimeters in length. The 

 basilar length, premaxillaries to occipital condyles, 

 ranges from 660 to 685 millimeters, the male zygo- 

 matic arches from 515 to 565, the horns from 155 to 

 200. In the females the dental and cranial measure- 

 ments exhibit the usual sexual contrasts of inferiority. 



Main features of skull: The nasals are short but 

 broad and thick in section in the type of B. validus, 

 85 to 103 millimeters in length and 100 to 143 milli- 

 meters in breadth; in other males (Nat. Mus. 4245, 

 4703, 4706) they are somewhat long and narrow, 

 resembling those in B. hracTiycepTialus and thus afford- 

 ing a complete transition, as may be readily seen in 

 Figure 419. The adult or growth changes are 

 prophetic of the higher specific stages of this phylum. 

 Thus in early stages of growth the nasals are some- 

 what thinner and more narrow distally; in later stages 

 of growth they thicken and broaden near the ends, 

 while the median cleft deepens. The form of the 

 horns is highly distinctive of this species; seen from 

 in front (PI. XCIV) the lateral contours of the horns 

 exhibit a strong convexity above the middle portion; 

 from this point upward they suddenly contract into 

 rounded, rugose tips; this rounding and pointing of 

 the tips of the horns is a very distinctive feature as 

 compared with the broad oval tips in members of the 

 Brontotherium phylum. The basal section of the 

 horn in the type skull of B. dispar (figs. 399, 419) 

 approaches that of B. rohustus (fig. 421) in the round- 

 ing out of the "anterior" or "nasal" angle or ridge 

 and in the anteroposterior flattening; the posterior 

 faces of the horns of B. dispar, however, unlike those 

 in the type of B. rohustus, are directed obliquely 

 inward toward each other and present a long, uniform 

 convexity. Some horn sections are more trihedral 

 because of the prominence of the nasal ridge. In 

 lateral view the horns are seen to be slightly recurved 

 and to be united, especially in the old males, by a 

 deep and broad connecting crest. The zygomata 

 present prominent convex buccal processes, which 

 have a deep, thick section, similar to that of B. 

 rohustus but somewhat less heavy. 



Details of male skull: The type skull of Brontops 

 validus, "skull K," Nat. Mus. 4290 (Pis. LXXXVIII, 

 LXXXIX, XCI, XCII; fig. 419), found at the 62-foot 

 level above the base of the Titanotherium zone, in 

 middle B, admirably exemplifies the distinctive 

 features of the skull. It is in the seventh stage of 

 growth. Seen from in front (PI. XCIV), the nasals 

 are supported by a vertical bony septum, which in 

 life may have been united by cartilage with a similar 

 bony septum that extends up from the premaxillae 

 (cf. Tapirella hairdi). The top of the skull is seen 

 to be a larger development of the B. hrachycepTialus 

 type (compare Pis. LXXXV and XCI), broad above 

 the orbits, narrowing at the vertex, with a rugose 



supratemporal crest. In the middle of the parietal 

 vertex of certain specimens (Nat. Mus. 4703) a very 

 large central pit (PI. XCII) is observed. This is a 

 vestige of the space between the convergent supra- 

 temporal ridges and is similar to the median pit in 

 some of the Eocene forerunners of Brontops — namely, 

 Manteoceras. The occipital pillars are strong, but 

 the rugosities at the summits are somewhat narrower 

 than in B. rohustus. The palatal view (Pis. 

 LXXXVIII, LXXXIX) also illustrates the brachy- 

 cephalic character. The vomer extends back as a 

 prominent keel upon the basisphenoid, which exhibits 

 a robust, paired rugosity for the rectus capitis muscle 

 at its junction with the basioccipital. 



The progressive size evolution of the skull is dis- 

 tinguished first by the slight increase in length and 

 great increase in width over that of B. hrachycepTialus, 

 owing partly to the rapid expansion of the buccal 

 processes but also to the broadening of the skull 

 itself; second, by the elongation and strengthening 

 of the horns as described above. Additional observa- 

 tions on this specimen are given above. 



Dentition: The teeth show many characters in- 

 herited from B. hracTiycepTialus. The formula (If^, 

 P|^) exhibits hereditary variability, manifested in 

 the occasional loss of one incisor and one premolar. 

 The median incisor sometimes disappears before the 

 seventh stage (Nat. Mus. 4941, 4703), or it may per- 

 sist late in life (Nat. Mus. 4290, 4245). The canines 

 are relatively long and rounded in section at the base, 

 more or less compressed anteroposteriorly, foreshadow- 

 ing the Allops sublanceolate canine, less elongate than 

 in members of the Menodus phylum, and much less 

 obtuse and recurved than in members of the Bronto- 

 tTierium series. The first superior premolar is a fairly 

 large and persistent tooth, but it also sometimes 

 aborts and disappears early (Nat. Mus. 4703). The 

 ectoloph of p' is well in line with the ectoloph of p^, p^ 

 (contrast BronfotTierium) . In the advanced stages of 

 B. dispar the transformation of the premolars has 

 proceeded much further than in any specimen of 

 B. hracTiycepTialus; the tetartocones of p^ and p^ are 

 relatively distinct, but in p* the tetartocone is still a 

 backward spur from the deuterocone, more rounded 

 than in B. rohustus, but never sharply rounded off or 

 prominent and always a much less distinct and sepa- 

 rate cusp than in p'. In the type of B. dispar, in fact, 

 the tetartocone of p* is only a little more advanced 

 than in the typical B. hracTiycepTialus. The external 

 cingulum of the superior premolars is fairly well 

 marked; the internal cingulum is progressive or 

 stronger than in B. hracTiycepTialus. On m^ the hypo- 

 cone is a prominent cingule. 



Lower jaw (fig. 423): In the type jaw (Nat. Mus. 

 4941) vertical crushing has greatly diminished the 

 depth of the horizontal ramus, but the symphyseal 

 region was probably shallow. There was a diastema 

 between pi (which has dropped out) and the canine. 



