EVOLUTION OF THE SKULL AND TEETH OP EOCENE TITANOTHERES 



331 



with the submolariform shape of the corresponding- 

 upper teeth, the third and fourth lower deciduous pre- 

 molars (dps and dp4) are more molariform than their 

 successors ps, p4, especially in having large posterior 

 lobes with high entoconids, which latter are lacking in 

 the permanent teeth; (5) the second and third upper 

 deciduous premolars are quadricuspidate, not fully 

 quadrate, and may be described as sub- 

 molariform; (6) the fourth upper decid- 

 uous premolar (dp^) is fully molariform. 



Palaeosyops robustus (Marsh) 



Plates LV, LVI, LVIII, LXII; text figures 94, 

 284-288, 508-511, 521-523, 542, 544-546, 571, 

 685, 714, 718, 724, 737, 741, 760 



[For original description and type references see p. 161. For 

 skeletal characters see p. 626] 



Type locality and geologic Jiorizon. — 

 Bridger Basin, Wyo., especially the upper 

 exposures of Henrys Fork; Bridger forma- 

 tion, Vintaiherium - Manteoceras -Mesati- 

 rJiinus zone (Bridger D). 



Specific cliaracters. — Of massive breadth 

 and proportions; slightly more brachy- 

 cephalic than P. major and P. leidyi; meas- 

 urements (all estimated), total length of 

 skull, 440 millimeters; basilar length, 440; 

 zygomatic breadth, 340; p^-m^, 163-170; 

 mesostyles variable on p^, more constant 

 on p*; molars with oblique ectolophs; 

 m^ enlarged, with prominent parastyle, 

 ectoloph oblique, molar conules strong; 

 distinctly rugose frontonasal horn 

 swellings. 



This stage is less perfectly known than 

 P. leidyi, our knowledge being confined 

 to the structure of the cranium, of the 

 superior dentition, of a few of the ver- 

 tebrae and of numerous fragmentary limb 

 bones. 



Materials. — The type maxillary teeth 

 are in the Yale University collection (No. 

 11122). In the American Museum collec- 

 tion the following are the principal speci- 

 mens: No. 11683, Bridger level D 3, ca- 

 nines to m^, progressive, close to P. robustus, 

 type; No. 1580, crushed skull with rudi- 

 mentary horns and associated skeletal 

 fragments, from Henrys Fork; No. 1554, 

 skull with rudimentary horns, same stage as type; No. 

 11678, Bridger level D 4, a broad skull with smaller 

 teeth; No. 5106, Cope's type of Palaeosyops diaconus, 

 from Henrys Fork, progressive; Princeton Mus. 

 10282b, maxillary from Henrys Fork. Also the fol- 

 lowing less progressive forms: Am. Mus. 1584, palate 

 and teeth; Am. Mus. 1552, palate and teeth, from 



Twin Buttes, level Bridger C or D; Am. Mus. 1558, 

 also Twin Buttes, level Bridger C or D, palate with 

 p^-m^; Am. Mus. 1586, i'-m^ unprogressive; and Am. 

 Mus. 1590, fragments of lower jaw, mi_3. We still 

 lack the complete lower jaw. 



General specific cliaracters of P. robustus. — The horn 

 swellings, as compared with those of three of the skulls 



Figure 281. — Skulls of Palaeosyops leidyi and P. copei 



robustus) 



One-fourth natural size. Ai, P. leidyi, Am. IVlus. 1581; Henrys Fork Hill, Bridger Basin, Wyo.; Bridger 

 formation, level probably upper C or D; old male skull, side view, showing rugose horn swelling 

 (purposely emphasized in the drawing); skull straightened from Am. Mus. 1544. As, The same; top 

 view of the region of the horn swelling. B, P. copeil, Am Mus. 12205a; Lone Tree, Henrys Fork, 

 Bridger Basin; level Bridger D 1; top view of skull. 



described under P. leidyi, are here more strongly 

 developed. The upper grinding series averages 6 

 millimeters longer than that of P. leidyi. The depth 

 of the ramus behind ma is estimated at 98 millimeters. 

 A metatrophic character is the relatively large size of 

 m^ measuring 42 millimeters transversely in the type, 

 exhibiting large conules and more distinct external 



