638 



TIT.\NOTHERES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



Figure 560. — Atlas of Mesati- 

 rhinus megarhinus 



These unassociated bones agree very well with those of 

 Am. Mus. 1571. In Bridger C 5 was found Am. Mus. 

 11659, including the radius, ulna, manus, femur, tibia, 

 and pes, which afford a knowledge of the relative pro- 

 portions of the fore and hind limbs. These bones are 

 considerably larger than 

 those of any of the other 

 specimens and appar- 

 ently pertain to a large 

 individual of M. petersoni. 

 In beds of the same age 

 (Washakie A) was found 

 the leading associated 

 specimen (Am. Mus. 

 1571), including the front 



Am. Mus. 1523; upper Bridger. The ar- . r .i 1 11 A 



rows indicate the course of the first spinal part 01 the SKUll and. Up- 



nerve and vertebral artery. Dorsalview. pgp dcnjtition of MeSOti- 



One-third natural size. , . , • j j.i 



rh'inus petersom, together 

 with the radius, ulna, incomplete manus, astragalus, 

 and fragments of the pelvis. 



GENERAL CHAKACTERS OF THE SKELETON OF MESATIRHINUS 

 AS COMPARED WITH OTHER EOCENE TITANOTHERES 



Atlas relatively narrow and deep, elongate, facets 

 for axis facing inward. Humerus relatively shorter, 

 radius relatively longer than in Palaeosyops or Man- 

 teoceras; radius long, slender, with narrow extremities 

 proximal, and distal articular facets shallow antero- 

 posteriorly. Ulna slender, with erect, subtruncate 

 olecranon. Manus high and narrow, straight-sided 

 parallel metacarpals, and short phalanges; carpals in 

 general narrow, vertically deep. Scaphoid shallow 

 anteroposteriorly, broad posteriorly. Lunar narrow, 

 lower end sharply wedge-shaped; magnum facet sub- 

 vertical; cuneiform facet ^^ ,.-., ,.-.... 

 broad; cuneiform narrow, 

 not extended postero-exter- 

 nally. Pisiform with prox- 

 imal end sharply con- 

 stricted from the shaft. 

 Trapezium small, pear 

 shaped, articulating with 

 scaphoid. Trapezoid wide 

 anteroposteriorly, shallow, 

 truncate posteriorly. Mag- 

 num small, scaphoid facet j^' cpom. ^^^ 

 broad. Unciform narrow, ^.^^^^^ 561.-Humerus of 

 vertically deep, lunar facet Mesatirhinus megarhinus 

 quadrate. Metacarpals Am. ivius. 12385; Bridger c or d. a,, 

 with carpal facets shallow 

 and truncate (rather than 

 extended backward as in Palaeosyops), elongate, 

 straight -sided; fifth metacarpal long and narrow, 

 proximal end embracing unciform externally. Prox- 

 imal phalanges relatively shorter than in Limno- 

 hyops and Palaeosyops, longer than in Manteoceras; 

 distal phalanx of D. Ill widely spreading and sharply 



Front view; As, outer side view. 

 One-sixth natural size. 



truncate distally, longer than in Manteoceras, median 

 cleft fairly marked. Femur slender with prominent 

 trochanters, with patella facet oblique to long axis of 

 shaft. Tibia long and slender, about y^ of the length 

 of the femur, with deeply concave facet for astragalus. 

 Pes with high tarsals, slightly divergent digits, narrow, 

 straight-sided metatarsals. Astragalus relatively high 

 and narrow, with narrow trochlea and elongate neck, 

 cuboid facet narrow, sustentacular facet narrow, ver- 

 tically elongate. Calcaneum elongate with narrow 

 sustentaculum and deep, laterally compressed tuber. 

 Navicular relatively deep vertically, forming a quad- 

 rant in superior view. Entocuneiform vertically oval, 

 pointed at top. Mesocuneiform anteroposteriorly 

 elongate, narrow. Ectocuneiform relatively deep, 

 shallow anteroposteriorly, and narrow transversely. 

 Cuboid deep vertically, shallow anteroposteriorly, 

 with subquadrate superior and inferior facets. Meta- 

 tarsals long, straight sided, 

 distal facet of Mts III 

 transversely cylindrical, 

 proximal facets truncate 

 posteriorly (not deep as in 

 Palaeosyopinae). Phalanges: 

 Proximal phalanx of Mts III 

 relatively . long, narrow; 

 middle phalanx wide and 

 shallow ; distal phalanx rela- 

 tively long, very broad 

 distally, sides obliquely 

 truncate. 



The atlas (Am. Mus. 1523) 

 shows (fig. 560) a subfamily Figure 562 

 agreement with that of 

 Manteoceras and with that 

 of DolicJiorJiinus and con- 

 trasts with that of Palaeo- 

 syops in the following char- 

 acters: The cotylus is narrow transversely (93 mm.) 

 but deep vertically (40 mm.); its superior border 

 is deeply concave anteriorly; the neural arch is 

 produced above into a circular rather than an elon- 

 gate hillock; the neural tunnel is more roundly 

 arched above; the postero-median inferior process 

 below the odontoid is acuminate, not peg-shaped; 

 the axis facets do not diverge so widely — that is, 

 they face more inward; the pleurapophysial flanges 

 are not preserved; the vertebrarterial canal was large. 

 The atlas is distinguished from that of Manteoceras by 

 its relative narrowness and depth, both anteroposte- 

 riorly and vertically. 



A humerus (Am. Mus. 12385) provisionally referred 



to Mesatirhinus megavMnus from Bridger C 3 agrees 



in most characters with that of Manteoceras (Am. Mus. 



12204) but is smaller and more slender. In length 



i (260 mm.) it is far shorter than the humerus of 



-pr.sti'.rd, 



Xz 



Radius and 

 ulna of Mesatirhinus 

 petersoni 



Am. Mus. 1571; Washakie A. Ai, 

 Right ulna, outer side view; A2, 

 right radius and ulna, front view. 

 One-sixth natural size. 



