EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



519 



cephalic, the zygomatic index being 62 to 70. They 

 are distinguished also by the extreme reduction 

 within the incisive alveoli of vestiges of the upper 

 incisor teeth, a fact which is confirmed in every speci- 

 men, whereas in Allops we find two small but per- 

 sistent upper incisors which pierce tha gum. In the 

 lower jaws the incisors appear to be wanting entirely, 

 although the evidence is less conclusive. As distin- 

 guished from Brontops the premolar teeth are very 



directed, and adapted to lateral motions of the head in 

 defense or attack; (3) persistently elongate nasals, 

 the only phylum in which this character occurs; (4) 

 vigorous development of the grindiag teeth, including 

 a high dental index, a strong development of the cin- 

 gula, and a marked vertical elongation or subhypso- 

 donty of the crowns; (5) the elongate and piercing 

 character of the canine tusks, which project so far 

 above and below the line of the grinders that it is 



A B 



Figure 435. — Sections and contours of skulls of Menodus heloceras and M. trigonoceras 



A, M. heloceras, Am. Mus. 6360 (type) ; small horns trihedral in basal section, parietal crest narrow, zygomata very slender. B, M. trigonoceras, Am. Mus. 6355 

 (lectotype or type?); pointed pyramidal horns roundly trihedral in basal section, connecting crest pronounced, nasals long and wide, parietal vertex 

 wide, and zygomata very slender. C, AT. trigonoceras (or giganieusf), Nat. Mus. 1219; horns longer with trihedral section, nasals very wide and distally 

 expanded. One-seventh natural size. 



rapidly progressive in the evolution of the postero- 

 internal cusps or tetartocones. This character is also 

 shared by Allops. 



Characters oj proportion. — -The six great distinctive 

 characters of Menodus are correlated with its length 

 of limb, height of body, and relatively cursorial habit, 

 as follows: (1) Dolichocephaly or length of skull con- 

 trolling all the parts both of the bones and of the teeth; 

 (2) short, triangular, and pointed horns, outward 

 101959— 29— VOL 1 36 



reasonable to regard them as weapons of defense and 

 offense; (6) the entire absence of incisor teeth, a 

 feature in which Menodus parallels Megacerops and 

 Brontotherium. Of these sLx characters the elonga- 

 tion of the skull (mesaticephaly or dolichocephaly) is 

 the most dominant in correlation with all parts of the 

 skull and teeth. 



The dolichocephaly of Menodus is demonstrated in 

 every measurement in the table below. In length 



