KINETOGENESIS. 



291 



From early Tertiary times to the present day, we can 

 trace the gradual development of this groove in all 

 the lines which have acquired it. The upper surface 

 became first a little concave ; 

 the concavity gradually became 

 deeper, and finally formed a well- 

 marked groove. 



The history of the wrist-joint is 

 similar. The surface of the fore- 

 arm bones which joins the fore- 

 foot is in the early Tertiary Mam- 

 malia uniformly concave. In the 

 ruminating mammals it is divided 

 into three fossae, which are sep- 

 arated by sharp keels. These fos- 

 sae correspond with the three 

 bones which form the first row of 

 the carpus or palm. The keels 

 correspond to the free sutures be- 

 tween them. The process has been 

 evidently similar to that which has 

 been described above as produc- 

 ing the side-grooves in the end of 

 the tibia. The dense walls of the 

 sides of the three bones imping- 

 ing endwise on the broad yielding 



surface of the fore-arm (radius) bones of' two middle toes 

 have gradually, under the influence f deer - antel P* <&**?* 



' Jurcatus Leidy), showing 



Of COUntleSS bloWS, impressed extension of keel on front 



themselves into the latter. On of lower end (From Mio- 



cene of Nebraska. 



the contrary, the surface above 



the weaker lines between the bones not having been 



subject to the impact of the blows, and influenced by 



Fig. 71. United first 



