GREGORY, QUADRUPEDAL LOCOMOTION 28? 



forms that progress by bounding have quite long femora. In this con- 

 nection, it must be remembered (p. 278) that a long femur, implying 

 small angles of insertion of the principal extensors, gives relatively high 

 speed of rotation of the insertion points, but low power for the rotation 

 components. The long femur of graviportal forms has a wholly different 

 meaning (p. 289). 



Progressive reduction of the side toes in the Artiodactyla as in the 

 Perissodactyla is accompanied by the elongation of the cannon bone 

 (here represented by two coalesced digits, metatarsals III. and IV.) and 

 by other cursorial adaptations. The primitive Artiodactyl foot with 

 four complete side toes is represented in Oreodon, which has a ratio of 

 .38. In Sus the ratio is .34, in the primitive four-toed camel Eotylopus 

 reedi it rises to .52, in Tragulus to .66, whence it rises rapidly to 1.00 

 and more in the deer and antelopes, culminating in 1.35, an extreme 

 figure, in the giraffe. In the giraffe, the development of great body 

 size has not brought about any reduction in the length of the metacar- 

 pals, but, in correlation with the long neck, has even lengthened them. 



Other adaptive contrasts in the feet. — There are other adaptive con- 

 trasts in the feet of graviportal and cursorial animals, as follows: 



In graviportal forms, the astragalus is flattened down, for, when the 

 weight of the body is raised, it is easier to force the tibia up a gentle 

 slope than a steep one. No lateral keels are needed on the trochlear; 

 surface to prevent dislocation because of the breadth and spreading- 

 character of the tarsus and of the large size of the fibular malleolus. In 

 cursorial animals, on the contrary, the curvature of the astragalar troch- 

 lear is steep and the range of movement wide ; the trochlea keels help to 

 keep the narrow tarsus in place. 



With regard to the phalanges, in cursorial animals, the sharp bending 

 at the fetlock and pasterns and the sudden straightening out of these 

 joints under the pull of the powerful flexors of the foot greatly assists in 

 projecting the body into the air (Stillman, loc. cit., p. 89). 



In graviportal animals, on the other hand, the terminal phalanges are 

 reduced ; the massive flexors of the foot raise the weight slowly and assist, 

 the animal in rolling from one foot to the other. 



