-42 



TITAN OTHEKES OF ANCIENT WYOMING, DAKOTA, AND NEBRASKA 



The distal end of the humerus is thus highly dis- 

 tinctive as between cursorial or graviportal function. 

 It exhibits in the mediportal Tapirus a balanced con- 



FiGTJRE 672. — Forms of distal articular surface of humerus 



A, Primitive (,Phenacodus); B, primitive (Pantolamida); C, mediportal (Tapirus); D, 

 subgraviportal (Limnohyops); E, graviportal digitigrade iShinoceros); F, graviportal 

 rectigrade ( Coryphodon); G, cursorial (Equus). All one-third natural size. 



dition of muscular attachment upon the entocondyle 

 and ectocondyle; similarly the deltoid crests are 

 moderately developed and the tuberosities are high. 



A marked characteristic of graviportal adaptation 

 is the rapid development of the ectocondylar crest 

 and of the extensor carpi radialis muscle; this develop- 

 ment is initiated in Palaeosyops and is carried to an 

 extreme in Rhinoceros and the later titanotheres. 

 The entocondylar process diminishes, and the 

 humeral trochleae are oblique, asymmetrical. 



In cursorial types like the Equidae neither the 

 entocondyle nor ectocondyle relatively increases, 

 obviously because the lateral or pronating and 

 supinating movements are almost wholly replaced 

 by orthal or extensor and flexor movements. In 

 cursorial forms the humeral trochlear convexities 

 become vertical; in graviportal forms they remain 

 oblique. 



ANALOGOUS ADAPTATION IN HUMERUS AND FEMUR 



Reviewing the following series of comparative 

 figures and descriptions of the limbs of the ungulates, 

 and especially of the perissodactyls, we deduce 

 certain laws of transformation and transposition of 

 areas of muscidar attachment that are almost as 

 constant as the allometric linear changes of pro- 

 portion set forth in section 1 of this chapter. 



Analogous principles of cursorial and graviportal 

 adaptation operate in general in the fore and hind 

 limbs, but there are some exceptions. The most 

 striking contrast is observed in the muscular attach- 

 ments at the upper end of the humerus and femur 

 respectively. 



In graviportal progression the tuberosities of the 

 humerus are raised and expanded and the analogous 

 great trochanter of the femur is lowered and de- 

 pressed. Conversely, in cursorial progression the 

 tuberosities of the humerus are lowered and de- 

 pressed and the great trochanter of the femur is 

 raised and expanded. 

 The chief divergences and parallelisms between the 

 transformations of the humerus and femur are shown 

 in the accompanying table. 



Divergences and parallelisms in the transformation of the humerus and femur in ungulates 



