432 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Specific characters. — Skull mesaticephalic, length 

 440 millimeters; breadth 210 (estimated); cephalic 

 index 47. Facial region upturned; basicranial region 

 subdolichocephalic; premaxillary symphysis elongate; 

 infraorbital shelf absent; malar rounded; preorbital 

 region relatively abbreviate; occipital condyles rela- 

 tively narrow. P'-m', 168 millimeters ; large hypocone 

 on m' (?variable); marked upward flexure (cypto- 

 cephaly) of premolar series and of incisive border; 

 superior premolars somewhat progressive, with rudi- 

 mentary tetartocone rectigradations ; molars elevated, 

 with prominent protocones. 



Materials. — This peculiar animal, according to 

 O. A. Peterson's record, is geologically more recent 

 than B. ahhotti, since it occurs in Uinta B 2 in the 



Aj. 



Figure 362. — Type skull of Rhadinorhinus diploconus 

 One-fourth natural size. Am. MuS. 1863 (type); White River, Uinta Basin, Utah; Uinta B 2. Ai, Side 

 view, as partly reconstructed in 1895 by H. F. Osborn and R. Weber; drawing reversed. A2, Top view. 

 Later reconstructions of this skull are shown in Figure 364. 



Eobasileus-DolichorTiinus zone. The type skull (Am. 

 Mus. 1863) was discovered by Peterson in Uinta B 2 

 during the American Museum expedition of 1894. 

 Our knowledge is partly supplemented by another 

 skull (Am. Mus. 2055), also from Uinta B 2. 



History. — The specific name R. [Telmatotherium) 

 diploconus was assigned by Osborn in reference to 

 the large hypocone on the last superior molar of the 

 type specimen, a character which is lacking in R. 

 ahhotti, also in the second specimen from Uinta B 2 

 (Am. Mus. 2055). There is, therefore, some doubt 

 whether the hypocone on m^ is constant. In the 

 original description it was also stated that the naso- 

 frontal did not possess a horn; there seem to be no 

 certain indications of a horn swelling in the nasofrontal 

 region. The extremities of the nasals are wanting. 



The small size of the canines in circular section may 

 indicate that the type specimen was a female. The 

 premaxillary symphysis is decidedly longer and more 

 firmly united than in MesatirMnus megarhinus. 



Distinctive features. — In contrast with MesatirMnus 

 and DolichorJiinus we observe that the frontoparietal 

 profile is concave instead of convex; associated with 

 this is the distinctively upturned facial region of the 

 skull. The mesaticephalic proportions of the skull are 

 indicated by the moderate transverse breadth of the 

 zygomata, with an estimated width of 210 millimeters 

 as compared with the total length of 440 millimeters 

 from the symphysis to the condyles. Correlated with 

 this is the relative narrowness of the occipital condyles 

 as compared with those of M. megarhinus. The primi- 

 tive elongation of the sagittal crest is 

 comparable to that in M.fluviatilis and 

 M. earlei. The animal also agrees with 

 these species decidedly in the narrow- 

 ness and abbreviation of the preorbital 

 region. Like these animals, it may 

 be described as narrow-snouted (a 

 characteristic of Megacerops). The 

 narial notch is also deeply recessed, 

 so that there is only a short space at 

 the side between the notch and the 

 anterior border of the orbits. The 

 postorbital processes of the frontals 

 are very large. Beneath the orbit is 

 found one of the most distinctive char- 

 acters — namely, the simple, rounded 

 form of the malars, which is in wide 

 contrast to the oblique shelf of 

 Manteoceras or the broadly project- 

 ing shelf of all the other species of 

 MesatirMnus and Metarhinus. 



R. diploconus differs from Metarhinus 

 fl,uviatilis as foUows: (1) All the cheek 

 teeth are more elongate anteroposteri- 

 orly, hence the internal border of the 

 molars is less oblique than in M.fluvi- 

 atilis; (2) the internal cingulum of p* is 

 not complete; (3) the postero-external 

 shoulder on p"* is more prominent; (4) the skull top in 

 side view is broadly concave (flatter in M. fluviatilis) ; 

 (5) the zygomatic arch in side view curves downward 

 more sharply. 



Variations. — Of the two skulls referred to R. diplo- 

 conus, one (Am. Mus. 2055) is smaller and has smaller 

 teeth than the type and lacks the hypocone on m'. 



Cyptocephaly . — The upturned face of R. diploconus 

 suggests that of Megacerops of the Oligocene. It 

 would seem that the skull in correlation was some- 

 what saddle-shaped above, with the nasal region 

 more elevated than the frontal. 



Features in detail. — The superior view of the type 

 skidl (Am. Mus. 1863) lacks the extremities of the 

 nasals; it exhibits the marked backward extension of 

 these bones, the great prominence of the orbital ring, 



