EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHEKES 



691 



lateral processes and with a higher neural protuber- 

 ance; the posterior ventral process is produced sharply 

 backward. The axis of B. leidyi (Carnegie Mus. 114), 

 apart from its much smaller size, differs from that of 

 B. robustus chiefly in the far less backward prolonga- 

 tion of the neural spine, in the more vertical position 



Figure 624. — Humeri of Brontops robustus and Brontotherium 

 leidyi 



A, Brontops robustus, Yale Mus. 12048 (type). B, Brontotherium leidyi, Carnegie 

 Mus. 93, associated with skull. Ai, Bi, Left humerus, front view; A2, B2, left 

 humerus, outer side view. In Brontotherium leidyi the crest of the great tuberosity 

 is relatively larger and more widely expanded and the deltoid process is relatively 

 smaller. One-eighth natural size. 



of the posterior zygapophysial facet and the less 

 transverse position of the anterior condylar facets for 

 the atlas. 



Vertebrae referred to Brontotherium gigas, American Museum 



This material is associated with a skull (see p. 570) 

 and a pelvis (see p. 692). 



The third (?) cervical vertebra (B. gigas, Am. Mus. 

 492) has the centrum much shorter anteroposteriorly 

 (55 mm.) than in B. rohusfus (type) ; its neural arch is 

 very small and slender; the lateral transverse process 



is more broadly expanded distally; and the zygapo- 

 physial facets, both anterior and posterior, appear to 

 face more vertically than in B. robustus. Accordingly 

 the neck of B. gigas appears to have been shorter and 

 deeper than that of B. robustus. 



The third dorsal vertebra (B. gigas, Am. Mus. 492) 

 is much stouter than that of Brontops robustus (type). 



Figure 625. — Humeri of Mcgaceroj. 

 acer? and Brontotherium gigas? 



A, M.i acer?. Am. Mus. 0351; B, B. gigas, Am. 

 Mus. 1062. Distal views. One-eighth natural 

 size. 



Its centrum is larger and deeper, the neural arch 

 longer, straighter, and wider. The swelling above the 

 facet for the tubercle of the rib is much larger. 



FiGUHB 626. — Radii of Brontops robustus, Brontotherium leidyi, 



and Brontotherium gigas 

 A, Brontops robustus, part of type skeleton, Yale Mus. 12048; B, Brontotherium 



leidyi, Carnegie Mus. 93, associated with skull; C, Brontotherium gigas. Am. 



Mus. 492, associated with skull. The two ends of the radius in Brontotherium 



appear to be wider, and the external contour more deeply concave; the styloid 



process is more acute. One-eighth natural size. 



The second(?) lumbar vertebra (B. gigas, Am. Mus. 

 492) has a less elongate centrum (ap. 87 mm.) than 

 that of Brontops robustus (type) (96 mm.); the neural 

 spine is apparently longer and more vertical and the 

 postzygapophysial facet more oblique; the anterior 

 face of the centrum is more convex. 



Fore limb referred to Brontotherium leidyi, Carnegie Museum 



This fore limb is from the Chadron A levels in 

 which the species B. leidyi occurs. 



The scapula of B. leidyi (Carnegie Mus. 93) is less 

 expanded transversely than that of B. gigas (Nat. 

 Mus. 4262). The generic differences from the scapula 



