434 



TITANOTHERBS OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Molars: M' (ap. 29 mm., tr. 27) is slightly longer 

 than broad and thus like the other molars is of marked 

 dolichocephalic type; the hypococone is very high and 

 prominent. M^ (ap. 37 mm., tr. 34) exhibits faint 

 external and incomplete internal cingula with a strong 

 antero-internal cingular basin, a progressive feature 

 of aU these molars. This basin, or "hypoconid fossa," 

 indicates relatively advanced hypsodonty in the lower 

 molars, which we might infer also from the subhypso- 

 donty of the upper molars. 



Peculiar also is the sharp furrow dividing the proto- 

 cones from the hypocones, and the prominence of the 

 internal cingulum midway between the protocones and 

 hypocones. M^ has an elevated ectoloph (28 mm.) 

 and is also longer (37 mm.) than broad (35 mm.); 



(3) nasals small, pointed anteriorly; (4) malars and 

 antorbital bar rounded, with very short space in front 

 of antorbital bar; (5) orbits small, inset; (6) a deep 

 recession at the sides of the nares, and nasals high set; 

 (7) occiput with a deep concavity; (8) chin weak, 

 concave below, sloping up to a plane higher than that of 

 the grinding teeth; (9) ramus with coronoid process, 

 etc. (see Am. Mus. 2059, Metarhinus fluviatilis, a 

 diminutive copy of the Oligocene Megacerops type); 

 (10) first and second upper incisors {R. ahhotti Riggs) 

 with short, rounded subcorneal crowns; (11) canines 

 of somewhat obtuse form, with swelling posterior 

 cingulum; (12) lower premolars sloping upward anteri- 

 orly with strong internal cingulum; (13) upper pre- 

 molars relatively progressive, well-rounded contours , 

 large tritocones and relatively 

 progressive tetartocone rudi- 

 ments; (14) tetartocone rudi- 

 ments in p^, p' {R. diploconus) 

 more advanced than in p*, pre- 

 molars sub quadrate in form; 

 (15) molars elongate or sub- 

 hypsodont. 



Intermediate forms between 

 R. diploconus which may be 

 discovered in Uinta B 2 and 

 Uinta C will determine the 

 question whether this relatively 

 primitive form is ancestral to 

 the Megaceropinae. 



successors to the manteo- 

 ceras-dolichorhinus group 

 (eotitanotherium, DIPLA- 

 codon) 



subfamily diplacodontinae (=?meno- 



DONTINAE, = ?BEONTOTHEKmf AE) 



[Eocene phylum Diplacodon] 



Including upper Eocene fore- 

 „ . , , , runners of the Oligocene genera 



One-fourth natural size. Am. Mus. 1863. White River, Uinta Basin, Utah; Umta B 2. Straightened and recon- ° "^ . 



structed, except nasals and condylar region. Ai, Side view (reversed); A2, front view, partly straightened, but JilenoduS, BrOntotlieriUm. 

 occiput probably too high and postorbital process of frontal too low; A3, occipital view, straightened. Primitivclv dolichocenhalic 



Figure 364. — Type skull of Rhadinorhinus diploconus 



its prominent hypocone constitutes a distinctive 

 feature, but from its absence in Am. Mus. 2055 it may 

 not be a valid and constant specific distinction. 



Is Rhadinorhinus ancestral to Megacerops? — The 

 possible ancestry of R. diploconus to Megacerops first 

 suggested itself to W. K. Gregory in 1903 and was 

 carefully considered by him and by Osborn in sub- 

 sequent years. There are, in fact, many features in 

 which Rhadinorhinus diploconus appears to fore- 

 shadow Megacerops, chief among which are the fol- 

 lowing: (1) Saddle-shaped profile, high nasofacial 

 region, depressed frontoparietal profile; (2) facial 

 region relatively abbreviate, upturned — that is, an 

 upward flexure of maxillaries and premaxillaries; 



progressively mesaticephalic. Precocious develop- 

 ment of horns. Nasals narrowing anteriorly, curved 

 downward at the tips. Second internal cones 

 on the superior premolars precociously developed. 

 (For progressive characters see Oligocene stages, 

 p. 467.) 



The phyletic relations of the two animals now to be 

 described are not certainly determined; they are pro- 

 visionally placed in a separate subfamily (Diplaco- 

 dontinae), although it is possible that they belong 

 in one of the OHgocene subfamiUes (Menodontinae, 

 Brontotheriinae). They include the Diplacodon of 

 Marsh, discovered in 1873, and the Eotitanotherium of 

 Peterson, discovered in 1912. 



