376 



MECHANICAL EVOLUTION. 



Constant jarring on the lower extremity of a hollow cylinder with 

 soft (medullary) contents and flexible end walls, would tend to a 

 decurvature of both inferior and superior adjacent end walls. If 



the side walls are wide and resistant, the 

 projection will be median, and will be 

 prolonged in the direction of the flexure 

 of the joint. The groove of the astraga- 

 lus deepens coincidently with the increase 

 of digitigradism, showing that, while it 



is primarily pro- 

 duced by shocks, 

 it is extended 

 fore and aft by 

 the repeated flex- 

 ure of the ankle- 

 joint. (2) They 



Fig. 7'J. 



Fig. 80. 



Fio. 81. 



Fig. 79, Part of anterior foot of Proeamelus occidentalis from New Mexico. Orig- 

 inal from Eeport of Capt. G. M. Wheeler, vol. iv, Ft. ii. 



Fi<i. 80, Metacarpals of Cosoryx furcatus from Nebraska, two thirds natural size, 

 a, anterior face ; J, posterior ; c, proximal end ; 6?, distal end. 



Fig. 81, Left fore foot with part of radius of Poehrotheriimi vilsoni Leidy, from 

 Colorado, three fitlhs natural size. From Hay den's Eeport, iv. 



fit entering grooves of the proximal ends of corresponding bones. 

 These will be the result of the same ajiplication of force and 



