EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



433 



the gentle supratemporal ridges converging from the 

 very prominent postorbital processes into the long, 

 narrow sagittal crest, the gentle outward arching of 

 the zygomata. In the badly distorted palatal view of 

 the same skull a resemblance to Mesatirhinus is never- 

 theless evident. The distorted occiput exhibits an 

 elevation of 97 millimeters and a deep superior con- 

 cavity. In the lateral view (figs. 362, 364) we are 

 struck by the downward, midcranial curvature of the 

 profile (which appears more marked than in Mesati- 

 rJiinus and Manteoceras) , by the very prominent 

 conformation of the orbital 

 region above described, by 

 the narrow space, measuring 

 only 45 millimeters, between 

 the orbit and the narial notch. 

 The malar, as in other titano- 

 theres, forms the entire outer 

 portion of the infraorbital 

 bar, the maxillary entering 

 into its anterior rim, as in 

 MesatirTiinus and Manteo- 

 ceras. It appears to lack 

 the infraorbital shelf as noted 

 above. The malar passes 

 anteriorly into the narrow 

 and decidedly convex and 

 elevated bridge at the point 

 of union with the lacrimals. 

 It is narrow and flattened 

 below the posterior portion 

 of the orbit as it passes 

 backward; it is thin on the 

 inferior surface. The zygo- 

 matic portion of the squa- 

 mosal is moderately elevated 

 and expanded. 



Dentition. — The materials 

 consist of the type skull (Am. 

 Mus. 1863) and of another 

 skull (Am. Mus. 2055) con- 

 taining alveoli of the cut- 

 ting teeth and the worn crowns of the grinding 

 teeth. 



Incisors: The superior incisor alveoli have the 

 characteristic convergence of the opposite series, and so 

 far as can be judged from the alveoli, which increase 

 regularly from i' to i^, the canine fang (ap. 14 mm., tr. 

 12) is laterally compressed and is of small dimensions, 

 like those of a female. 



The molar-premolar series are of the Mesatirhinus 

 type; they measure 169 millimeters in length, as com- 

 pared with 156 in M. petersoni. They are distinguished 

 by several features, among them the very prominent 

 pointed hypocone of m^, to which the specific name 

 refers but which is lacking in Am. Mus. 2055. P^, p' 



exhibit the very rudimentary postero-internal or 

 tetartocone ridges which are observed in T. ultimum 

 and Dolichorhinus. There are other faintly indicated 

 progressive characters, especially the comparatively 

 deep medifossettes and postfossettes on the molars 

 and premolars, correlated with the greater hypso- 

 donty, and the nearly symmetrical protocone and 

 tritocone convexities on the ectoloph of p^"'*, as in M. 

 earlei. 



Premolars: More in detail: P' is separated from 

 the canine by a narrow diastema (5 mm.) and is a 



f:/,l.a.(n. 



Type skull of Rhadinorhinus diploconus 



One-fourth natural size. Am. Mus. 1863. White River, Uinta Basin, Utah; Uinta B 2. Partly straightened, 

 occipital crest has heen narrowed by crushing. Ai, Top view; M, palatal view. 



bifanged tooth; p^ is in a much more progressive 

 stage of evolution than in M. petersoni, with its sub- 

 quadrate form, anteriorly placed deuterocone, sub- 

 functional tetartocone, well developed and slightly 

 convex tritocone, the dimensions (ap. by tr.) being 16 

 by 19 millimeters; p^ is of similar pattern (ap. 17 

 mm., tr. 21), with nearly symmetrical protocones and 

 tritocones, an anterior style, and a rudimentary pos- 

 tero-internal cusp or tetartocone. In the develop- 

 ment of the last-named cusp p^ is slightly more pro- 

 gressive than p^ (ap. 20 mm., tr. 25); p^ is of the same 

 pattern but entirely lacks any trace of the tetartocone. 

 An internal cingulum nearly surrounds these teeth ex- 

 cept opposite the deuterocone. 



