334 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



but by comparison with other specimens this cuspule 

 is seen to be certainly a metaconule and not a true 

 hypocone. 



A similar " pseudo-hypocone " condition of the meta- 

 conule on m^ is also observed in a skull from Henrys 



Figure 286. — Hyperbrachycephalic old male skull 



robustus 

 One-fourth natural size. Am. Mus. 1580; Henrys Fork, Bridger Basin, Wyo.; Bridger formation. 



conules, prominent parastyles, little or no postcanine 

 diastema. (See fig. 286.) 



The close concurrence of measurements and pro- 

 gressive and retrogressive characters in the above- 

 mentioned specimens with those of the type of P. 

 rohustus fully establishes this species as a dis- 

 tinct stage of evolution. 



Jaws. — The jaws of this species are not yet 

 fully known. There are portions of the rami 

 of the young adult (Am. Mus. 1590) contain- 

 ing mi_3, which measure 119 millimeters (esti- 

 mated), as compared with 107 in P. leidyi. 

 In these molars the cingulum is progressive and 

 the paraconids are large. These teeth appear 

 to correspond with the P. rohustus stage. 



Sicull of P. rohustus. — Three skulls are at- 

 tributed to this species in the American Mu- 

 seum collection, namely, Nos. 11678, 1554, and 

 1580. 



Horns. — The matter of chief interest is the 

 structure of the osseous horn knobs. Kudi- 

 mentary frontal horns are seen on Am. Mus. 

 1554 (fig. 285), which are even more prominent 

 than in the transitional skull Am. Mus. 1581 

 attributed to P. leidyi. The position of the 

 protuberances is on the frontals behind the 

 nasal sutures; they are a little more posterior 

 in position than the rugosities observed in P. 

 leidyi, the center of the protuberances being 18 

 millimeters behind the suture and 59 milli- 

 meters from the median or internasal suture. 

 The convexity of the horn is a complete oval, 

 approximately 29 millimeters in diameter in 

 transverse and longitudinal sections (PI. XVI). 

 The frontal bones are thickened and more can- 

 cellous beneath the horn. 



In a very aged and robust skull. Am. Mus. 1580 



Palaeosyops 



probably level D. Skull crushed downward. Shows horn swelling (?i) and extreme rugosity, fr- 286) which is COVCrcd with CXOStOSCS the horn 



Fork (Am. Mus. 11678), level Bridger D 4; this tooth, 

 like that in Cope's type of P. diaconus, entirely lacks 

 the true hypocone. Although this skull is a male its 

 molar-premolar series is relatively short, the three true 

 molars measuring only 94 millimeters, as compared 

 with 101 in the type of P. rohustus. Mesostyles on 

 p'"^ if originally present are worn away. Progressive 

 features are the marked external cingula of the molars. 

 This important skull is illustrated in Figure 287. 



A skull that exhibits rudimentary horns (Am. Mus. 

 1554) shows in its detailed measurements (p^, trans- 

 verse, 28 mm. (estimated); m^, transverse, 40; m', 

 oblique, 52) close approximation to the type measure- 

 ments. The mesostyle is absent on p", wanting or 

 worn off on p^ (See fig. 285.) 



A more robust skull with horns (Am. Mus. 1580), 

 Henrys Fork, level Bridger D?, exhibits measurements 

 (p^ 28 mm.; m^ transverse, 41; m^ oblique, 52) 

 which are very close to those of the type; also large 



knobs are more prominent and rugose and still more 



Figure 287. — Basicranial region of Palaeosyops rohustus 



One-fourth natural size. Am. Mus. 11678; Henrys Fork, Lone Tree, Bridger Basin, 

 Wyo.; Bridger formation, level D 4. 



posterior in position than in the specimen above 

 described, the center being 23 millimeters back of the 

 frontonasal suture. 



