EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 



639 



Pdaeosyops leidyi (325 mm.). The supinator crest is i Palaeosyops sp. (35). The external proximal facet for 



relativeh" shorter. It is sharply separated from the 



Ai Az 



FiGUKE 563. — Left forearm and manus of Mesa- 

 tirhinus petersoni? 



Princeton Mus. 10013; Bridger C or D. Ai, Front view; A2, 

 outer side view. One sixth natural size. 



humerus of the contemporary cursorial rhinoceros 

 Hyrachyus by the marked asymmetry of the radial 

 facet, the marked down- 

 ward extension of the 

 deltoid ridge, the stouter 

 supinator crest, and the 

 heavier proximal end. It 

 is smaller than even the 

 smallest (Am. Mus. 1571) 

 of the three forearms 

 referred to Mesafirhinus 

 petersoni. 



The radius of M. peter- 

 son (Am. Mus. 1571, 

 Princeton Mus. 10013) is 

 remarkable for its length 

 (233-245 mm.), its slen- 

 derness (circumference of 

 shaft just above middle, 

 72-75), and the narrow- 

 ness of its proximal end 

 (tr. 49, 50) and distal end 

 (tr. 49, £0). By way of 

 comparison, in Palaeo- 

 FiGURE 564. — Right manus and syops copei (Am. Mus. 

 fragments of radius and ulna 12205) the corresponding 

 of Mesaiirhinus petersoni measurements are length 



Am. Mus. 1571; Washakie A. A, Right 907 -iv ± -u jxi, 



manus; B, distal end of radius and ulna. ^'^' nUJuniCters, Dreaatll 



Facets for scaphoid (sc), lunar (lu), and proximal end 62, distal 



cuneiform (cu) . One-third natural size. j „►, • r r 



end 67, circumference of 

 shaft 87. The proximal end of the radius of M. 

 petersoni is also shallower (ap. 30 mm.) than in 

 101059— 29— VOL 1 14 



the capitellum of the humerus is much shallower 

 anteroposteriorly (20 mm.), more deeply concave 



Figure 565. — Left manus, radius, and ulna 

 of Mesaiirhinus petersoni 



Princeton Mus. 10013; upper Bridger. Ai, Front view of 

 manus; Az, top view of carpus; As, inner side view of car- 

 pus; Bi, radius and ulna, distal view; B2, radius, proximal 

 view. One-third natural size. 



anteriorly, and continues externally into a high 

 anteroposterior ridge which is higher than that in 

 Palaeosyops. This ridge is supported interiorly by the 

 angulate external border 

 of the shaft, which slopes 

 downward and inward at 

 a gentle angle, whereas in 

 Palaeosyops the rounded 

 external border rapidly con- 

 tracts into the shaft so that 

 the proximal end of the 

 bone expands widely. The 

 shaft of the radius arches 

 forward a little less strong- 

 ly than in uncrushed speci- 

 mens of Palaeosyops, but, 

 as already noted, is much 

 more slender. The distal 

 end is much less expanded 

 transversely and relatively 

 deeper anteroposteriorly; 

 the internal distal process 

 is sharply prolonged down- 

 ward. The radius of M. petersoni (Am. Mus. 11659) 

 is badly crushed but differs from those above de- 



FiGURE 566. — Right scaphoid 

 of Mesatirhinus and Man- 

 teoceras 



Front and top surfaces. Ai, As, Mesa- 

 iirhinus petersoni, Princeton Mus. 

 10013; Bi, B2, Manieoceras manteo- 

 ceras, Am. Mus. 12204. One-half 

 natural size. 



