EVOLUTION OF THE SKULL AND DENTITION OF OLIGOCENE TITANOTHERES 



445 



oval extends into the sides of the nasals (N); the pos- 

 terior edge of the oval subsides laterally toward the 

 frontals, lacrimals, and maxillaries {M). 



Second stage of develop- 

 ment. — A low transverse 

 " connecting crest " arises 

 at the median bases of 

 the hornsand gives them 

 a more or less distinct tri- 

 lateral section consisting 

 of an antero-internal face, 

 an antero-external face, 

 and a posterior face. 

 These three faces are es- 

 pecially distinct in spe- 

 cies of Menodus, such as 

 M. trigonoceras . In Mega- 

 cerops, however, the con- 

 necting crest is not typi- 

 cally developed, so that no 

 internal angle (C) appears 

 {Megacerops acer, p. 545). 

 Sections of the horn. — 

 Each phylum takes on a 

 characteristic horn sec- 

 tion, which may be repro- 

 duced by bending a cop- 

 per or lead wire in the Figure 375 

 manner shown in Figure 

 376. The horn section is 

 always cut at right angles 

 to the perpendicular axis 

 of the horn and as near 

 the base of the horn as 

 practicable. Each horn 

 develops progressively a 

 characteristic section at 

 the base (fig. 399) and at 

 the tip. The four chief 

 types of horn sections 

 are as follows: 



Short-horned: 



Anteroposterior oval, subtriangular, oblique oval, 



subtransverse oval : Brontops, Diploclonus. 

 Subtriangular oblique, subtriangular transverse: 

 Menodus, Allops. 

 Long-horned: 



Subtriangular, suboval, transverse oval: Bronto- 



therium. 

 Rounded, anteroposterior oval, flattened anteri- 

 orly, rounded posteriorly: Megacerops. 

 Forward shifting of horns. — The horns tend to shift 

 forward and absorb the nasals. Thus the general 

 correlation of horns and nasals is as follows: 



Short, triangular, oval horns, elongated nasals; Bron- 

 tops, Menodus. 

 Long, oval or rounded horns, abbreviated nasals: Bron- 

 totherium, Megacerops. 



-Sections at base of 

 horn in the six chief generic 

 types of Oligocene titanotheres 

 (B-G) and in the upper Eo- 

 cene Prolilanolherium emargi- 

 natum (A) 



A, ProtitanoiheriumemaTginatum, Princeton 

 Mus. 11242; anteroposteriorly oval. B, 

 Brontops dispar, Nat. Mus. 4290; rounded, 

 obliquely oval. C, Diploclonus amplus, 

 Yale Mus. 12015a (type); rounded, trans- 

 versely widened, trihedral. D, Allops 

 serotinus, Yale Mus. 42.')1 (type); angu- 

 late, trihedral. E, Menodus giganteus. 

 Am. Mus. 505 (neotype); rounded, trihe- 

 dral. F, Megacerops acer, Am. Mus. 

 6348 (type): rounded, quadrate. G, 

 Brontotkeriitm plutyceras, Harvard Mus. 

 (type); transversely oval. All one-fifth 

 natural size. 



In the extremely long-horned types, such as Bronto- 

 therium, the horns shift forward until they overhang 

 the anterior nares and finaUy the symphysis; they 

 thus absorb the nasals but retain their base of sup- 

 port on the greatly shortened maxillaries. Thus the 

 nasal angle (iV) disappears, and the horns acquire a 

 transverse oval section. 



Horns in females. — In skulls from the higher geologic 

 levels of the Oligocene the difference between the 

 horns of the two sexes is rather marked; in skulls from 

 the lower Oligocene and from the Eocene the difference 

 is less. There is reason to believe that the horns were 

 at first alike in both sexes. In females the horns 

 exhibit an arrested stage of development. This fact 

 is most clearly shown by a comparison of two female 

 skulls of Brontotherium (Am. Mus. 1005, 1006) with 

 two male skulls (Am. Mus. 492, 1070). (See fig. 377.) 

 In many females the horns are imperfectly ossified at 

 the tips; in some they are pointed. In species of 

 Menodus the "connecting crest" is more constant and 

 more pronounced in males than in females, but in 

 females of Brontotherium the connecting crest appears 



Figure 376. — Position of the standard sec- 

 tions and contours of Oligocene titanothere 

 skulls 



N, Nasal contour; M,median section of nasals and connect- 

 ing crest; H, basal horn section; HN, oblique-longitud- 

 inal section, nasals to horn tip; P, section across parietal 

 verte.x; B, buccal section of zygoma. 



to rise almost to the summit of the horn, as in the 

 female skull selected by Cope as the type of Menodus 

 peltoceras ( = Brontotherium curtum) (fig. 478). The 



