752 



TITANOTHERES OF ANCIENT Wl'OMING, DAKOTA, AND NEBRASKA 



ceras), in which the scapula shortens and the humerus 

 elongates; not so in the Old World rhinoceroses {R. 

 (Opsiceros) bicornis, R. unicornis), in which the 

 scapula exceeds the humerus in length; (2) the hume- 

 roradial ratio increases in Teleoceras and Metamynodon 

 onh' — that is, the radiohumeral ratio decreases; (3) in 

 the Old World rhinoceroses the radius remains rela- 

 tirelj^ of the same length as the humerus; (4) the neck 

 ("collum") of the scapula broadens; (5) the postero- 

 superior border of the scapula becomes depressed 

 (American graviportal genera only); (6) the infra- 



This series parallels the progression characteristic 

 of graviportal titanotheres in certain features but not 

 in others. 



This series represents a sequence of adaptive types 

 but not a phylogenetic sequence. It includes mem- 

 bers of four subfamily types, namely, hyracodont 

 (subcursorial), aceratheriine (mediportal), amyno- 

 dont (graviportal-digitigrade), and rhinocerotine (gra- 

 viportal-digitigrade). The comparison between the 

 cursorial, mediportal, and graviportal limbs exhibits 

 the following laws of progression from subcursorial 



Figure 682. — Limb structure of perissodactyls: Hind limbs of rhinoceroses found in the Old World 

 A, CeratoThinus sumatrensis: B, Opsiceros (Diceros) iicornis; C, Hhinoccros indkus: D, E.javanicus. One-tenth natural size. 



spinous fossa of the scapula increases {Metamynodon, 

 Teleoceras); (7) the supra- and infraspinous fossae are 

 equal or balanced in the heavy Old World types 

 (Rhinoceros, Opsiceros); (8) the tuber spinae of the 

 scapula increases and points upward; (9) the great 

 tuberosity of the humerus rises above the head; (10) 

 the deltoid crest is lowered to the middle of the 

 shaft and rises in prominent tuberosities; (11) the 

 ectocondylar crest of the humerus rises on the shaft, 

 the entocondylar crest disappears; (12) the ulna 

 olecranon becomes recumbent and expanded distally. 



and mediportal to graviportal adaptation: First the 

 tibia relatively shortens, thus the femorotibial ratio 

 increases, or the tibiofemoral ratio decreases; the 

 shaft of the femur broadens, flattens, and straightens 

 {Amynodon, Teleoceras, Rhinoceros, Opsiceros); the 

 great trochanter, which is narrow and elevated in 

 cursorial hyracodonts, becomes broad and depressed 

 in the graviportal amynodonts, in Teleoceras, and in 

 Rhinoceros; the third trochanter shifts from above 

 downward to the middle part of the shaft and becomes 

 very prominent (the rhinoceroses are unique in this 



