MECHANICS OF LOCOMOTION 



753 



character); the second trochanter remains on the 

 upper part of the shaft and becomes less prominent; 

 the femoropatellar facet becomes more anterior 

 (Metamynodon) as the limb straightens out; the 

 patellar facet rises on the front of the shaft of the 

 femur as the knee becomes less flexed and the hind 

 limb more vertical; the same facet becomes more 



which show signs of adaptation to increased weight. 

 The cursorial maximum is reached in the lower 

 Pliocene Hipparion {NeoJiipparion) , after which there 

 is a retrogression to the more heavy-bodied Equus 

 and Hippidium. 



It is striking to observe how early in geologic time 

 certain cursorial characters are attained in the horse 



Figure 683. — Limb structure of perissodactyls : Fore limbs of Equidae, showing their cursorial adaptation 



A, Orokippus osbornianiis, two-sevenths natural size. B, Mesohippus hairdi, one-fifth natural size. C, Hypoliippns osborni; T>, Neohipparion 

 xoliitneyi; E, Hipparion gracile; all one-seventh natural size. F, Equus kiang; G, Eguus burckdli: H, Eguus caballus; all one-ninth natural 

 size. Scales approximate. After De Blainville, 1839.1, and Osborn. 



. asymmetrical (a progressive syngenetic but not dis- 

 tinctively a graviportal character). 



EQUIDAE 



The accompanying figures (683, 684) represent 

 individuals selected from polyphyletic series of Eciuidae 

 beginning with the primitive Eocene cursorial and 

 light-bodied types, including the Oligocene cursorials 

 and the extremely cursorial Neohipparion, the Plio- 

 cene Hipparion, and the modern heavy-bodied Equus, 



family. Among the fore limb cursorial adaptations 

 of Eocene equines are (1) scapula with relatively small 

 supraspinous fossa (as in cursorial Artiodactyla) ; (2) 

 with narrow "collum scapulae"; (3) with fan-shaped 

 blade; (4) ulna with elongate vertical olecranon. A 

 persistent primitive character is the acromial rugosity 

 of the spine of the scapula. 



Additional cursorial adaptations acquired after 

 Eocene time are (1) reduction of tuber spinae scapu- 

 lae; (2) scapulohumeral ratio increase; (3) humero- 



