430 



TITANOTHERES OF ANCIENT "WYOMING, DAKOTA, AND NEBRASKA 



giceps in Uinta B 1 . It is a well-defined branch from 

 the more typical Dolichorhininae and, as first observed 

 by Gregory, has the right combination of characters 

 for a remote ancestor of the Megaceropinae. Such an 

 ancestral relationship, however, awaits confirmation 

 by discovery in Uinta C. RJiadinorJiinus certainly 

 does not lead either into Diplacodon or into Protitano- 

 therium. Riggs observes (1912.1, p. 41): 



Rhadinorhinus apparently represents a side branch from the 

 early Metarhinus stem. The facial and cranial regions and the 

 zygomata are similar. The nasals indicate an early specializa- 

 tion in another direction. The dentition is more highly spe- 

 cialized than that of Dolichorhinus and in some respects 

 resembles it. The posterior nares open a little farther forward 

 than those of Metarhinus. The mere trace of an infraorbital 

 process also removes it somewhat from the long-nosed, low- 

 ground titanotheres. 



The grounds for relating this phylum to that of 

 Megacerops { = Symborodon) are stated fully below. 



DolicTiorhinine affinities. — Eemote relationship to 

 the dolichorhines {MesatirTiinus-DolichorMnus) rather 

 than to Manteoceras is indicated by resemblance in 

 the following characters: (a) Form of temporal and 

 sagittal crests; (6) form of zygomatic arches in top 

 view; (c) marked preorbital concavity and projecting 

 orbits; (d) slight projection of infraorbital portions 

 of malar; (e) similar conformation of palatal and 

 basicranial region; (/) rudiments of a secondary palate; 

 (g) incisors somewhat resembling the dolichorhiue 

 type; (h) premolars and molars in all features doli- 

 chorhine with added peculiarities. 



These animals differ widely from the typical doli- 

 chorhines, however, in the relatively short, pointed 

 nasals and in the concave or saddle-shaped profile of 

 the cranium as seen from the side but even more 

 distinctly in the upbending of the grinding series, 

 which carries with it an upturning of the anterior 

 portion of the face, a feature very distinctive of 

 Brontoiherium and especially of Megacerops, as shown 

 in Figures 401-403. 



Rhadinorhinus Riggs, 1912 



Plates LXXIV, LXXX, LXXXII; text figures 109, 141, 322, 

 359, 362-364, 401-403, 405, 647 



[For original description and type references see p. 192] 



Geologic horizon. — MetarJiinus zone (Uinta B 1); 

 also Eohasileus-Dolichorhinus zone (Uinta B 2). 



Generic characters. — Titanotheres with slender sub- 

 dolichocephalic skulls; cephalic indices 47-52; nares 

 deeply recessed laterally; orbits not prominent; nasals 

 tapering or pointed and abbreviate anteriorly; in- 

 fraorbital shelves rudimentary or wanting. Molars 

 with elevated hypocones; premolars somewhat pro- 

 gressives; hypocone of m' present or rudimentary. 



History. — As shown above the first of these animals 

 known was originally described by Osborn in 1895 as 

 " TelmatotTierium diploconum," but it was subsequently 



(1908.318) transferred by the same author to Meta- 

 rhinus diploconus. The type lacked the nasal bones. 

 The superior specimen found by Riggs, in 1910, a 

 skull with the nasals preserved, justified his creating 

 the new genus and species Rhadinorhinus abhotti, 

 belonging to Uinta B 1 , or a lower geologic level than 

 that of R. diploconus (Uinta B 2). 



The species appear to be distinguished as follows : 



Rhadinorhinus abhotti Riggs, Uinta B 1; skulls somewhat more 

 primitive and of smaller size; length 426 millimeters, breadth 

 224, cephalic index 52; hypocone on m' vestigial. 



Rhadinorhinus diploconus (Osborn), skulls of somewhat larger 

 size, length 440 millimeters, breadth 210, cephalic index 47; 

 hypocone on m' large. 



The indices of these species, which are of limited 

 reliance because based on single specimens, are re- 

 markably similar, as follows : 



Indices of Rhadinorhinus 



Rhadinorhinus abbotti Riggs 



Plate LXXX; text figures 141, 403 

 [For original description and type references see p. 192] 



Type locality and geologic horizon. — Uinta Basin, 

 Utah; Metarhinus zone (Uinta B 1). Rare. 



Specific characters. — Skull mesaticephalic, length 426 

 millimeters, breadth 224; cephalic index 52. P^-m^ 

 164 millimeters; m'-m^ 104. Nasals shorter than 

 premaxillaries, thickened at suture, and tapering 

 toward a terminal rugosity; posterior nares opposite 

 m^; sagittal crest long and narrow. First and second 

 incisors with semioonical crowns; postcanine diastema 

 short. 



Materials. — The type of this interesting species 

 (Field Mus. 12179) was discovered by Mr. J. B. 

 Abbott, in whose honor it was named by Riggs. No 

 other specimens are recorded from Uinta B 1 . 



From the author's original description the following 

 citations may be made: In its general proportions the 

 skull of R. ahhotti (PI. LXXX) is similar to that of 

 Metarhinus riparius; somewhat longer and more 

 slender in the zygomatic arches; the skull suggests a 

 longer-limbed and more active animal; the skull is 

 slight in construction, the arches more slender; the 

 frontal region is rounded; the postorbital processes 

 elongate; the nasals are convex on the inferior surface 

 owing to the thickening at the sutural line; the facial 

 and palatal aspects differ widely from those of Meta- 

 rhinus riparius i the anterior narial opening is high and 

 the nasals are not infolded at the sides; they terminate 



