EVOLUTION OF THE SKULL AND TEETH OF EOCENE •TITANOTHBRES 



389 



The material in the American Museum referred to 

 this species includes the following: From the Bridger 

 Basin, skull, No. 12202 (level C 5); palates, Nos. 12206 

 (level C 5), 1519, 1523 (level unknown); lower jaws, 

 Nos. 1520, 1551, 12207 (level C 5), 12199 (level C 5); 

 from the Washakie Basin (level A), palates, Nos. 1513, 

 1514; lower jaws, Nos. 1575, 1577. 



Mesadrhinus petersoni Osborn 



Plates L, LXXI, LXXII; text figures 26, 27, 33, 122, 211, 217, 

 219, 255, 302, 327-331, 333, 339-341, 483, 611-514, 516, 520- 

 523, 526, 558, 559, 562-571, 586, 647, 656, 661, 686, 702, 713, 

 716, 724, 745 



[For original description and type references see p. 182. For 

 skeletal characters see p. 641] 



Type locality and geologic Jiorizon. — ■ 

 Cattail Springs, Bridger Basin, Wyo.; 

 Uintatherium-Manteoceras-MesatirMnus 

 zone (Bridger), level D 3. Also re- 

 corded from Bridger C (?) and 

 Washakie A. 



Specific cliaracters. — P^-m^, 154-157 

 millimeters; true molars 87-90. Skull 

 length, premaxillaries to condyles (esti- 

 mated), 402-425 millimeters; cephalic 

 index, 51-52; preorbital facial region 

 (estimated), 195-205 millimeters; facio- 

 cephalic index 48. Other characters 

 as in M. megarhinus — that is, broad 

 occipital condyles, infraorbital shelf, 

 etc. 



Materials. — The type skull (Am. 

 Mus. 12184) is from Bridger D 3 (fig 

 327). Comparison of this animal with 

 the type of M. megarhinus can leave 

 no doubt that we have to do here 

 with a more advanced stage of evolu- 

 tion. The skull is longer, the preorbital 

 region especially. The grinding teeth 

 occupy more space, and there is an 

 average advance in all the premolar 

 rectigradations, which prove that these 

 differences in form and size are not 

 merely due to fluctuations of size or 

 differences of sex. 



Other specimens (in the American 

 Museum except as noted) referred to 

 this species are, from the Bridger, skulls Nos. 1509 

 (level D) and 1556 (level D; now in British Museum), 

 lower jaw No. 1567, lower jaw No. 12191 (level C 2); 

 from Washakie A, skull No. 1571 and lower jaws 

 Nos. 1512, 13178. 



Of these No. 1571, from Washakie A, fortunately 

 has associated with it the fore foot, radius, ulna, 

 astragalus, and pelvis. Another valuable skeleton 

 (Am. Mus. 11659) is recorded from Bridger C 5, 



and a well-preserved forearm and manus in the Prince- 

 ton Museum (No. 10013) came from Bridger C or D 

 of Henrys Fork, Wyo. 



From Washakie A comes a very progressive doli- 

 chocephalic specimen (Am. Mus. 1651) consisting of 

 the three upper molars, which are strongly compressed 

 laterally and measure collectively 96 millimeters. 

 This specimen is provisionally referred to this species 

 and appears to be an important and interesting 

 transitional form leading into DolicJiorJiinus. 



General characters of Mesatirhinus megarhinus and 

 M. petersoni. — It is impracticable to describe M. 



Figure 326. — Type skull of Mesatirhinus megarhinus 



One-fourth natural size. Princeton Mus. 10008, Washakie Basin, Wyo., level Washakie A?. A i. Side 

 view (reversed) ; As, palatal view; A3, top view. 



megarhinus and M. petersoni separately, because it 

 would involve duplication of description. 



Sexual characters: Unfortunately the imperfect 

 preservation of the canine teeth does not admit of the 

 sharp separation of males and females that is possible 

 for many of the series of skulls. Comparison of the 

 teeth in the more perfectly preserved jaws, however, 

 shows that the canines were decidedly smaller in the 

 females than in the males. 



