360 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



and rugose frontonasal horns; (3) progressive anterior 

 flattening of the vertex of the cranium and recession of 

 the sagittal crest; (4) presence of a supraparietal pit 

 and strongly bifid sagittal crest; (5) broad and de- 

 pressed occiput; (6) oblique shelf-like suborbital part 

 of the malars; (7) widely arched zygomata with de- 

 scending flange of malar relatively shallow; (8) pro- 

 gressively more round-topped superior incisors, the 

 outer relatively smaller than in Telmatherium; (9) 

 shorter, heavier, and rounder superior canines, with 

 very heavy roots; (10) less progressive tritocones, deu- 

 terocones, and cingula on superior premolars; (11) 

 somewhat less pronounced hypsodonty of the grinding 

 teeth; (12) broader and more rounded mesostyles; 

 (13) the less deep and finally more elongate premax- 

 illary symphysis. 



Comparisons with MesatirTiinus and DolichorTiinus. — 

 The general resemblances of these animals have been 

 enumerated above. A number of resemblances in 

 general conformation are seen by comparison of 

 similar views of the crania of Manteoceras and of 

 Mesatirhinus. These indicate a closer ancestral af- 

 finity to MesatirTiinus than to Telmatherium. De- 

 tailed points of resemblance between Manteoceras and 

 Mesatirhinus are seen in (1) the tendency to form a 

 suborbital shelf, which is more pronounced in Mesati- 

 rhinus than in Manteoceras; (2) the depth of the facial 

 concavities, giving prominence to the nasofrontal 

 horn rudiments (a distinction must be noted here, 

 however, that the horn rudiments in Mesatirhinus 

 and Dolichorhinus are borne rather by the nasals than 

 by the frontals, whereas in Manteoceras the reverse is 

 the case); (3) pronounced affinities in the foot and 

 limb structure. 



The statement may be made very emphatically, 

 therefore, that Manteoceras and Mesatirhinus have 

 risen from a common stock. 



The distinctive characters of Manteoceras lie prin- 

 cipally in the proportions of the skull, dentition, and 

 feet and in the divergent evolution of the premolar- 

 molar series. Manteoceras is mesaticephalic in skull 

 and tooth structure and subbrachypodal in foot 

 structure, while Mesatirhinus is progressively both 

 dolichocephalic and dolichopodal. 



Incipient horns. — As observed above, a notable 

 characteristic of these animals is the precocious horn 

 development. The horn swellings are borne directly 

 over the frontonasal suture (Pis. XVI, XVII). They 

 involve very slight convexity and are slightly rugose 

 only in the more aged specimens. As they are 

 exhibited in various degrees in all the skulls known, 

 they were certainly present in both sexes, although less 



prominent in the females. These horn swellings have 

 a different origin in Dolichorhinus, as well as in Mesati- 

 rhinus, for in these genera (PI. XVII, figs. B, C\ C), 

 although placed about as in Manteoceras, they are borne 

 chiefly on the nasals and partly on the frontals — that 

 is, in front of the frontonasal suture. 



Facial concavities. — The second distinctive character 

 that is correlated with or lends itself to this precocious 

 development of the horns is the concavity in the side 

 of the face, in front of the orbit, beneath the nasal. 

 This gives a greater prominence to the horn rudiments 

 and in life would permit the warty epidermal swellings 

 that covered these rudiments to be used more ef- 

 fectively in butting. This overhanging frontonasal 

 suture showB a wide contrast to the condition seen in 

 Palaeosyops. The concavity of the face in front of the 

 orbit, beneath the horn, is a very prominent feature 

 also in the Oligocene titanotheres and in Sthenodectes 

 incisivus of level B of the Uinta Basin, Utah. 



The vertex. — In lateral or profile view the skull is 

 convex above the brain region, concave in the mid- 

 cranial region, and convex again in the nasal region, 

 as in Mesatirhinus. The horn rudiments, or hornlets, 

 are thus thrown into considerable prominence both 

 laterally and superiorly. The concave midportion of 

 the skull is again a progression in the direction of the 

 saddle-shaped top of the titanothere cranium. When 

 viewed from above the cranium also exhibits a spread- 

 ing of the space beneath the supratemporal ridges in 

 such a manner that the sagittal crest proper is limited 

 to the posterior region. In the V-shaped space on 

 top of the skuU between these converging temporal 

 ridges (or bifid sagittal crest) it is especially interesting 

 to observe that a deep pit is developed in the more 

 progressive and older forms, because we shall find a 

 vestige or reversion to this pit on top of the large, 

 flattened crania of some of the Oligocene titanotheres 

 {Brontops, compare figs. 304, 307, 374). 



Dentition. — The superior incisors form a more 

 A-shaped series than in Dolichorhinus, where they 

 tend to form a fl, and the inferior incisors are more 

 transverse in position. The grinders are less hypso- 

 dont on the ectoloph, and the protocone tips are more 

 blunt than in Mesatirhinus and Dolichorinus. The 

 premolars are less advanced than in Mesatirhinus and 

 Dolichorhinus, because the tritocones and deuterocones 

 are relatively smaller, the ectolophs less flat, and the 

 "ribs" on the external face of the outer cusps wider 

 at the base. The relative degree of progression of 

 the premolar ectolophs in Dolichorhinus and Manteo- 

 ceras is a very complex matter, but after careful 

 comparison it may be summarized as follows: 



