EVOLUTION OF THE SKULL AND TEETH OF EOCENE TITANOTHERES 



379 



There is a marked asymmetry of this tooth on the two 

 sides of the jaw, the right tooth being much smaller 

 than the left and of a form normal in upper Eocene 

 titanotheres. The abnormal pi is much swollen, a 

 tendency seen also in the other cheek teeth and even 

 more emphasized in the Oligocene titanotheres. The 

 normal pi (ap. 16 mm., tr. 11) is compressed, sub- 

 conic, with a posteriorbasal lobe that is better devel- 

 oped than in earlier forms. P2 (ap. 25 mm., tr. 16) is in 

 about the same stage of evolution as in P. superhum 

 but is more brachyodont; it exhibits a protoconid 

 relatively much more depressed than in M. manteoceras, 

 while the postero-external cusp (hypoconid) is rela- 

 tively more elevated, and a rudimentary internal cus- 

 pule ( = paraconid) begins to appear. This is therefore 

 a much more progressive tooth than the po of M. 

 manteoceras but is clearly derivable from it. In pg, 

 though the anterior crescent ( = trigonid) remains 

 larger than the posterior crescent ( = talonid), the 

 measurements (ap. by tr.) being 27 by 17 millimeters, 

 the antero-internal cusp begins to be well defined; no 

 median internal cusp corresponding to the metaconid 

 of the molars appears, the tooth being less progressive 

 in this respect than in P. leidyi but derivable from the 

 conditions observed in ps of M. manteoceras. P4 dif- 

 fers from the true molars chiefly in its smaller size (ap. 

 30 mm., tr. 21), in the somewhat greater elevation of 

 the anterior lobe ( = trigonid), and in the absence of a 

 distinct postero-internal cusp ( = entoconid) . The 

 external cingula are obsolete on p2, ps and much 

 reduced on P4. 



The inferior molars of the type are characterized by 

 shallow internal valleys and an incomplete external 

 cingulum, which is inflected in the valleys, by a well- 

 defined posterior cingulum, by prominent paraconid 

 and metaconid, by a considerable elevation (hypo- 

 conid) of the crown (26 mm.) in m2. Metaconid folds 

 are present as in many other titanotheres. The meas- 

 urements (ap. by tr.) are mi, 46 by 26 millimeters; 

 m2, 57 by 34 (estimated) . M3 is incomplete in the type ; 

 in another specimen (Am. Mus. 2028) it measures 

 78 millimeters, and in a third specimen (Yale Mus. 

 635 D) 79. In Protitanotherium superhum this tooth 

 measures 98 millimeters. 



Jaw of Protitanotlierimn emarginatum. — The type 

 jaw of P. emarginatum. (Princeton Mus. 11242) ex- 

 hibits the anterior half of the ramus and symphysis 

 The second jaw (Am. Mus. 2028) preserves the pos- 

 terior half but lacks the coronoid and condylar proc- 

 esses. The third jaw (Yale Mus. 635 D) includes 

 the symphyseal portion only. 



The type jaw exhibits a very massive symphysis, 

 159 millimeters in length, 80 millimeters across the 

 narrowest portion of the chin below, with the charac- 

 teristic postcanine constriction seen in M. manteoceras. 

 The ramus is thickened (35 mm.) below mi and in- 

 creases very rapidly in depth from 92 millimeters 

 behind ps to 126 behind m2. The progressive increase 

 of the ramus in depth posteriorly is evident in this 



series as well as in Telmatherium ultimum, T. altidens, 

 Palaeosyops, etc. It is more pronounced in forms with 

 relatively large molars. The mental foramen of the 

 type jaw is single and placed directly below the 

 posterior fang of p2. 



In the American Museum jaw of P. emarginatum 

 (No. 2028) the depth is 90+ millimeters behind pa^ 

 108 behind m,, and 144 behind ms; the distance from 

 the back of ms to the posterior border of the angle is 

 186 millimeters. The lower border is crushed, the 

 distortion concealing its natural contour, but there 

 appears to be a slight up curve below the coronoid, 

 with a broad downward and backward sweep of the 

 angle, and the posterior border of the angle appears 

 to rise to the condyle with a slight incurvature. 



Protitanotherium superhum Osborn 



Plate LXIX; text figures 128, 315, 318, 321, 371, 593, 6i7 

 701 



For original description and typs references see p. 185. For skeletal characters 

 see p. 655] 



Type locality and geologic horizon. — White River, 

 Uinta Basin, Utah; Uinta formation (Diplacodon- 

 Protitanotherium-EpiJiippus zone, Uinta C, probably 

 higher levels). 



Specific cTiaracters. — Very large (pi-ms, 318 mm.). 

 Lower canines in males very robust, relatively nearer 

 to the midline than in P. emarginatum; pi distinctly 

 double-fanged; postcanine diastema abbreviated 

 (about 30 mm.); premolars in about the same stage of 

 complication as in P. emarginatum, but premolar and 

 molar cusps more steeply sided (that is, more hypso- 

 dont), p4 submolariform, ps, p2 transitional; external 

 cingulum on ps, p4 a little clearer; true molars very 

 large (mj-ms, 210 mm.); ms with hypoconulid sharply 

 constricted at base. 



Materials. — This species is at present known from 

 the type jaw (Am. Mus. 2501), discovered by Peterson 

 in 1895. There are also two upper molar teeth re- 

 corded as belonging to the same individual, as well 

 as a pair of nasals in the Princeton collection (No. 

 11213). These nasals (fig. 318) are distinguished 

 from those of P. emarginatum by their superior size 

 and by a deep incision in the median line anteriorly. 



Comparisons. — The relative measurements of P. 

 superium, P. emarginatum, and Teleodus avus are 

 shown below. 



Measurements of Proiitanotherium and Teleodus, in millimeters 



