304 THE STRUCTURAL EVIDENCE OF EVOLUTION. 



there is not a single 

 instance in which those 

 bones are locked in the 

 Lower Eocene, as they 

 are in the later and 

 latest Tertiary. So 

 also with the articula- 

 tion of the toes with 

 the foot. 



When we come to 

 the limbs, the species 

 of the Puerco fauna 

 have short legs. They 

 have gradually length- 

 ened out, and in the 

 late periods they are 

 nearly all relatively 

 long. 



Coming to the ver- 

 tebrae as a part of the 

 osseous system, I men- 

 tion the zygapophyses, 

 or anteroposteriorly 

 directed processes, of 

 which the posterior 

 looks down and the an- 

 terior looks up. They 

 move on each other, as 

 the vertebral column 

 bends from side to 

 side. In the lower 

 forms of vertebrates 

 they are always flat, 

 and in the hoofed 

 mammals of the Puer- 

 co period they are all 

 flat. In the Wasatch 

 period we get a single 

 group in which the 

 articulation, instead of 

 being perfectly flat, be- 



