EVOLUTION OF THE SKELETON OP EOCENE AND OLIGOCENE TITANOTHERES 



595 



and a narrow, nearly vertical magnum facet, similar 

 to but relatively smaller than that seen in Mesatirhinus 

 megarJiinus; posteriorly it rests half on the magnum, half 

 on the unciform. As this'bone, therefore, practically 

 distributes three-fourths of 

 its front weight to the 

 unciform, it exhibits an ex- 

 treme displacement which 

 appears to be a character 

 common to the early Eocene 

 Perissodactyla generally. 

 Similarly the scaphoid is 

 narrow (13 mm.) and deep 

 anteroposteriorly (24 mm.) 

 with a vertical measure- 

 ment of 19 millimeters; it 

 covers the entire superior 

 face of the magnum. The 

 magnum is a very distinc- 

 tive bone as seen in front, 

 being small, vertically ex- 

 tended (12 mm.) and later- 

 ally compressed (9 mm.). 

 The imciform is a vertically 

 deep element measuring 22 

 by 19 millimeters trans- 

 versely. The metacarpal 

 displacement is also extreme, 

 Mtc II and Mtc III abut- 

 ting widely against the 

 magnum and unciform respectively. The metacar- 

 pals are distinguished by the small size of Mtc V, 



Figure 495. — Atlas of 

 Eotitanops borealis 

 Am. Mus. 14887 (neotype atlas asse- 

 dated with skull; cf. flg. 250). .^i. 

 Anterior or condylar surface; A2, 

 posterior or cervical surface; A3, dor 

 ^sal; Ai, ventral; As, side view. OnC' 

 third natural size. 



by 7 anteroposterior, being, therefore, relatively large- 

 Mtc III is still larger; it measures 85 millimeters 

 vertically and 13 transversely. Mtc IV measures 

 73 millimeters vertically and 11 transversely. Thus 



Figure 497. — Radius of 

 Eotitanops borealis 

 Am. Mus. 4892, fragments of the right 

 radius associated with the type upper 

 teeth. Ai, Distal part; A2, proximal 

 part; A3, proximal view. One-third 

 natural size. 



Figure 498. — Lunars of 

 Eotitanops 



Left lunar of JE. borealis (.\m. Mus. 

 4892, type; Ai, front view, A2, top 

 view) and E. princeps (Am. Mus. 

 296, type; Bi, front view, B2, top 

 view). 



Figure 496. — Vertebrae of Eotitanops princeps 



Cervical (Ce, C7) and dorsal (D) vertebrae associated with the type lower jaw 

 Mus. 296). One-half natural size. 



in which the shaft measures only 6 millimeters antero- 

 posteriorly by 9 transversely. Mtc II has a length of 

 79 millimeters and a shaft diameter of 12 transverse 



Mtc IV is decidedly smaller than Mtc II. This lack 

 of symmetry in the digits on either side of the third 

 digit is characteristic of the early Eocene titanothere 

 manus; it prepares the way for the secondary parax- 

 onic condition in which Mtc II, III, IV, V become 

 more nearly subequal. 



Humerus and femur 



The two limb bones known, the humerus and femur, 



are distinguished by the 



lateral compression of the 



shafts in harmony with 

 the compression of the 

 manus; they indicate 

 that E. princeps as com- 

 pared with the middle 

 Eocene titanotheres 

 was narrow chested and 

 slender limbed, that its 

 proportions most nearly 

 resemble those of Mesa- 

 tirhinus of the middle 

 Eocene, and that the 

 corresponding elements 

 are much more primi- 

 tive. 



The humerus (fig. 500) 

 is relatively elongate, a 



km. primitive character; the 

 great tuberosity is less 



developed than in more 



recent types. The shaft in the upper third measures 



20 millimeters transversely, 35 anteroposteriorly, as 



Figure 499. — Manus of 

 Eotitanops princeps 

 m. Mus. 296; Wind Eiver Basin, 

 Wyo.; Wind River formation, level 

 B. Eight manus of type. One-half 

 natural size. 



