EVOLUTION OF THE SKELETON OF EOCENE AND OLIGOCENE TITANOTHERES 



665 



on measurements. The calcanea of the opposite sides 

 are of entirely different shape, although there is little 

 external sign of crushing. Had these femora been 

 found in different localities they would certainly 

 have been referred to different species if not to 

 different genera. 



Comparison with Brontops rohustus. — As Hatcher 

 observes, not only is this skeleton smaller and some- 

 what less robust than that of B. rohustus, but there 

 are primitive characters of importance. (1) Com- 

 pare the slender, pointed neural spine of the first 

 dorsal vertebra, which although much longer resembles 

 that of the last cervical in form, with the broad, heavy 

 neural spine of the first dorsal in B. rohustus or in 

 Menodus. (2) In the carpus of B. dispar a well- 

 developed trapezium is present, while it is absent in 

 B. rohustus. In this connection Hatcher observes that 

 in all titanotheres from the lower Titanotherium 

 zone the trapezoid shows a trapezium facet. 



The measurements are given in the table above. 



Summary oj primitive characters. — This skeleton pre- 

 sents the following primitive characters as compared 

 with Brontops rohustus: (1) Less development of 

 neural spines on cervicals and dorsals, smaller neural 

 spines on cervicals; (2) relatively more elongate 

 scapula and innominate bones; (3) slightly less elevated 

 humeral tuberosity; (4) a trapezium; (5) lunar resting 

 more subequally on magnum and unciform; (6) 

 tarsus less compressed vertically; (7) less displace- 

 ment in manus and pes; (8) limited metacarpal dis- 

 placements — that is, Mtc II without large magnum 

 facet; (9) Mtc III with small ectocuneiform facet. 



Size and proportions. — The total linear proportions 

 of this skeleton to that of the Brontops rohustus? 

 skeleton in the American Museum (No. 518, fig. 606) 

 can not be given owing to the absence of the skull, 

 but a few comparisons can be made: 



Comparative measurements of Broniops dispar and B. robustusf 



Shoulder height, to spine 



of third dorsal 



Breadth of pelvis 



Length of fore limb, 



including scapula 



Length of hind limb 



Presacral vertebrae 



Length of femur 



Millimeters 



1,855 



935 



1,743 



1,500 



2,275 



686 



Ft. in. 



6 1 

 3 h 



5 9 



4 11 



7 5M 



2 3 



MillimeteTS 



2,300 

 1,220 



2,050 



1,670 



2,640 



790 



6 9 

 5 6 



The skeleton of B. rohustus is thus throughout 

 between one-sixth and one-eighth larger. 



Vertehrae. — The vertebral formula is C 7, D. 17, 

 L.?, S. 4, C. 18. The atlas (partly restored) exhibits 

 a hypophysial backward spur beneath the axis. 



More in detail, the fourth to seventh cervical ver- 

 tebrae resemble those of B. rohustus in the lateral 



processes but differ markedly in the comparatively 

 small spines on C. 6 and C. 7. D. 1 differs also in 

 its relatively slender spine; in D. 2-5 the spines are 

 approximately equal, that of D. 2 being the broadest 

 and somewhat the longest; in D. 7-14 the spines 

 gradually diminish in height; in D. 15 to L. 2 the 

 spines are relatively lower and broader than in 

 B. rohustus; in D. 3-12 the zygapophyses are hori- 

 zontal; in D. 13-17 the zygapophyses become obliquely 

 vertical. In L. 2 (the only lumbar preserved) the 

 transverse processes are rather narrow; the spine is 

 relatively broader and larger than in B. rohustus. 

 Twelve caudals are preserved, of which the third 

 bears a chevron bufnot the second. 



Rihs. — The first rib is less massive than in B. rohus- 

 tus. The ribs increase steadily in length from K. 1 

 to R. 8; R. 8 to R. 10 are the longest and of nearly 

 equal length; R. 11-17 steadily diminish in size. 



Sternals. — The manubrium is restored. S. 2 and 

 S. 3 are laterally compressed. The xiphisternum is 

 partially restored. 



Scapula and fore limh. — As compared with that of 

 Brontops rohustus the scapula is relatively higher and 

 narrower, the anterior and posterior borders being 

 more nearly parallel, and the general form more 

 quadrate than triangular; this is due to the relatively 

 larger supraspinous fossa, the long, straight anterior 

 border for the attachment of the levator anguli scapu- 

 lae, the inferior angle of which above the supracoracoid 

 notch is placed on a lower level than in B. rohustus; 

 similarly the infraspinous fossa for the infraspinatus 

 muscle is relatively smaller; as characterized by a very 

 prominent rugosity at the posterior superior angle 

 (latissimus dorsi), the superior or vertebral border 

 is less arche.d than in B. rohustus. 



The humerus exhibits a highly convex great tuber- 

 osity (supraspinatus and infraspinatus insertion), 

 which is relatively as elevated as in B. rohustus; the 

 deltoid ridge and tuberosity are equally prominent 

 and the external or supinator crest rises very promi- 

 nently, the shaft being deeply excavated between. 

 The internal face of the radius is somewhat less ele- 

 vated than in B. rohustus; its total length (404 mm.) 

 is double the circumference (202). The ulna exhibits 

 a broadly flattened anterior face; the olecranon lacks 

 the deep vertical cleft at the summit so characteristic 

 of the upper Oligocene titanotheres. 



The carpus shows the following characters: Breadth 

 167 millimeters, lunar resting broadly on the magnum 

 facet (28 to 32 mm.) as weU as on the unciform facet 

 (34 to 40 mm.), more primitive or less displaced than 

 in B. rohustus, much more primitive than in Menodus. 

 The trapezium is present; it is attached above to 

 the scaphoid but reaches only halfway down the inner 

 face to the trapezoid; it is not in contact at all with 

 Mtc II. Magnum broad; Mtc II with a narrow 

 magnum facet (20 mm.); Mtc III with a relatively 



