392 



TITANOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



of the lacrimals, the postorbital processes on the 

 frontals and malars, the slender malar portion of the 

 zygomatic arch, the comparatively slight upward 



Figure 329. — Skulls of Mesaiirhinus 

 petersoni 



One-fourth natural size. A, Front view of skull in 

 British Museum (formerly Am. Mus. 1566); Big 

 Bone Mountain, Henrys Fork, Bridger Basin, 

 Wyo., upper Bridger, level D, B, Occipital view 

 of Am. Mus. 1509; Big Bone Mountain, Henrys 

 Fork, Bridger Basin, upper Bridger, level D. 



cm'vature of the squamosal portion of the zygoma, the 

 incipient arching of the parietal region, the greatly 

 elongate and not deeply vertical temporal fossa, the 

 wide space between the postglenoid and post-tympanic 

 processes, the relations of the frontals, parietals, 

 occipitals, and squamosals, and the formation of the 

 temporal fossa. 



The occiput is relatively broad and low, measuring 

 (Princeton Mus. 10041, M. petersoni?) 107 millimeters 

 transversely by 85 vertically. There is a deep de- 

 pression in the superior portion of the occiput; the 

 occipital condyles are widely divergent superiorly 

 on either side of the foramen magnum. 



The anterior view of the skull of M. petersoni (fig. 

 329, A) best illustrates the characteristic form and 

 symphyseal junction of the premaxillaries, the stout 

 lateral decurvature of the nasals, and the postero- 

 lateral horn rudiments on these bones. 



Dentition; influence of dolichocephaly. — In general 

 the teeth show the dolichocephalic tendency, although 

 they have not reached the extreme of elongation seen 

 in the species of DolichorJiinus; they also are to be 

 contrasted with those of the more mesaticephalic 

 M. manteoceras. Thus it may be noted that Manteo- 

 ceras and Mesaiirhinus are separated by strong differ- 

 ences in the premolars and also in the molars. The 

 premolars of Mesatirhinus are distinguished from 

 those of Manteoceras as follows: (a) They are rela- 



tively longer as compared with their width; (b) in 

 crown view p^-p"* appear more circular than in Man- 

 teoceras in consequence of the deuterocones being 

 farther forward and the postero-internal part of the 

 crown more evenly rounded out; (c) the tritocones are, 

 on the whole, relatively larger and flatter externally; 

 (d) the external cingula are better defined opposite 

 the tritocone; (e) the protocone ribs on the ectoloph 

 are more pronounced and narrowed; (/) the protoco- 

 nules and tetartocones are better developed. Between 

 typical members of M. megarhinus and M. petersoni 

 the differences are of a progressive character — that is, 

 in M. petersoni the deuterocones and tritocones are 

 larger, the tetartocones and ectoloph ribs are much 

 more pronounced. 



Incisors. — The superior incisors (fig. 330) are ar- 

 ranged to form a forward-pointed arch — that is, the 

 opposite series are less parallel to each other than in 

 T. cultridens and less transverse in position than in 

 Palaeosyops. The series is short-crowned, with convex 

 anterior and convexo-concave posterior faces; the 

 posterior cingulum foreshadows the marked develop- 

 ment of the cingulum in DolicTiorhinus. A note- 

 worthy character is that i' is less caniniform than in 

 TelmatJierium. 



Canines. — The canines are subround in section 

 rather than laterally compressed as in TelmatTierium. 

 The enameled crown area measures vertically 36 milli- 

 meters and in base diameter 18 millimeters in certain 

 specimens of M. petersoni. In the smaller specimens 

 of M. megarhinus the crown measures 26 millimeters 



Figure 330. — Incisors, canines, and 

 prema.xillae of Mesatirhinus 



One-balf natural size. A, Crown view of ^f. 

 megarhinus, Am. Mus. 1514, La Clede Mead- 

 ows, Washakie Basin, Wyo.; B, side view of 

 M. petersoni, Am. Mus. 1671, Washakie Basin. 



in length. The inferior canines (Am. Mus. 1576, 

 1575) are more slender and rounder toward the tip, 

 with more feebly indicated anterior and posterior 



