EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



431 



above the anterior margins of the canines so that the 

 premaxillaries are exposed when the skull is viewed 

 from above. 



There is no antorbital fossa, as in M. fluviatilis; the 

 muzzle is broad and the canines evidently elongate. 

 The teeth are longer in the crowns (that is, more 

 hypsodont) throughout than in MetarMnus. The 

 author continues (1912.1, p. 37): 



The molar teeth are long in the crown, having inner cones 

 nearly equal in height to the eetoloph. The molar-premolar 

 series is well preserved and unworn in the type specimen. The 

 canines are broken at the alveolus; half of the incisors are pre- 

 served. The dentition as a whole is more highly specialized 

 than that of Metarhinus. The first and second incisors have 

 short, rounded, semiconical crowns indented by cups on the 

 posterior surface. The third incisor has a more elongate crown 

 (23 mm.), the cup is suggested by a prominent cingulum on 

 the posterior margin. An interval of 6 millimeters separates 

 the third incisor from the canine. The latter is 18 millimeters 

 in diameter and circular at the alveolar section. The elongate 

 third incisor would indicate a long-crowned canine. The first 

 premolar is a simple, blunt cone with an internal cingulum and 

 emplanted by two roots. Premolars ^' ^' * increase steadily in 

 length of eetoloph and deuterocone; strong internal cingula 

 persist. The last three have taken on the subrectangular out- 

 line indicating a stage in dental specialization similar to that of 

 Sthenodectes. The molars are long-crowned; the protocone 

 increases steadily in height from first to third. The hypocone 



is more prominent in the second and reduced to a vestige in the 

 cingulum of the third. The entire molar-premolar series is 

 more curved [that is, in a vertical plane] than in any other 

 Uinta titanothere. 



Additional observations on RTiadinorhinus ahhotfi. — A 

 fine skull in the Carnegie Museum (No. 2866, Uinta 

 B 1) has the following measurements, which are very 

 close to those of the type: 



Measurements of Rhadinorhinus abhotti, in millimeters 



Pmx to condyles 



Transverse zygomata 



Postorbital process to condyles 



Last molar to condyles 



p'-m' 



Mi-m3 



The infraorbital protuberance is small but distinct ; 

 the nasals are not so sharply pointed as in the type. 

 The superior maxillary symphysis is very long. This 

 specimen, together with the type, clearly shows that 

 Rhadinorhinus is an early offshoot from the Meta- 

 rhinus stem (W. K. Gregory). 



Measurements of skulls of Rhadinorhinus, Mesatirhinus, and Sthenodectes, in millimeters 



Length, incisors to condyles 



Breadth across arches 



Breadth between orbits 



Postorbital process to condyles 



Last molar to condyles ^. 



Length of free nasals ,__^j__^^ 



Greatest breadth of nasals 



Postglenoids to condyles (median line) - 



Breadth across condyles 



Greatest depth of arch 



Length of molar-premolar series 



Length of molar series 



Length of crown of canine . 



Diameter of crown of canine 



Length of diastema : 



Narrowest point in sagittal area 



Breadth of orbitonasal area 



R. abbotti, 

 Field Mus. 

 12179 (type) 



426 

 224 

 134 

 240 

 196 

 102 



84 



38 



164 



103 



R. diploconus, 

 Am. Mus. 

 1863 (type) 



440 

 '210 



172 

 110 



M. petersoni, 

 Am. Mus. 

 12184 (type) 



435 

 215 

 115 

 220 

 210 

 95 



165 

 105 



-20 

 10 



M. superior, 

 Field Mus. 

 12188 (type) 



585 

 240 

 120 

 276 

 260 

 138 



36 

 184 

 105 

 23 

 18 

 15 

 26 

 52 



S. incisivus, 



Carnegie Mus 



2398 (type) 



488 

 305 

 170 

 261 

 197 



207 



130 



56 



25 







94 



S. mcisivus, 



Field Mus. 



12168 



460 

 300 

 160 

 262 

 190 



101 



211 



132 



47 



27 







o Estimated. 



Measurements of lower jaw of Sthenodectes incisivus {Field 

 Mus. 12166) 



Millimeters 



Length, condyles to incisors 360 



Height, condyles above angle 168 



Length of molar-premolar series 215 



Length of molar series 130 



Length of crown of canine (estimated) 30 



Diameter of crown of canine 19 



Depth of ramus from base of ps 60 



Depth of ramus from base of ma 84 



Rhadinorhinus diploconus (Osborn) 



Plates LXXIV, LXXXII; text figures 109, 362-364, 401, 402, 

 405, 647 



[For original description and type references see p. 173] 



Tyfe locality and geologic horizon. — White River, 

 Uinta Basin, Utah; Eolasileus-Dolichorhinus zone 

 (Uinta B 2), two specimens. 



