568 THE MAMMALIA OF THE UINTA FORMATION. 



The rapid reduction of the fibula accompanying the orthal movement of the limb 

 hastens the displacement of the astragalus, in advance of any displacement in the 

 carpus. 



The growth of the asti'agalus upon the cuboid and the cuboid beneath the as- 

 tragalus are reciprocal. The former may be compared to the scaphoid and lunar dis- 

 placement, the latter to the growtli of the unciform beneath the lunar, i. e., towards 

 the mesaxial line. 



The line of association of reduction with certain forms of displacement observed 

 in the manus has a parallel in the astragalo-cuboidal disj^lacement. It will be ob- 

 served that in the artiodactyl or paraxonic types there was a primitive disposition to 

 form an astragalo-cuboidal facet, adapting the middle axis of the astragalus and 

 calcaneum to the middle axis of the metapodium, which here passes between the sec- 

 ond and third digits. On the other hand the Hyracotherium, Paloplotlierium and 

 IlyracodiM mesaxonic types, as they all tend to monodactylism, show an arrest of 

 displacement, owing to the growth of the third digit producing conditions similar to 

 those in the monodactyl carpus ; while genera of the iso-tridactyl mesaxonic type, 

 the tapir, i-hinoceros, etc., all show extreme astragalo-cuboidal displacement exactly 

 parallel to that in the carpus. 



Conclusions. 



The laws of adaptation of the serial plantigrade foot to digitigradism may be 

 summarized as follows : 



1. Displacement is effected by the gi'owth, arrested growth, or reduction of 

 different elements, and takes place in the direction of the greatest lateral strain, being 

 most rapid in the elements which are subjected to the maximum vertical impact and 

 lateral strain. 



2. The direction and degree of intercarpal displacements are adapted 

 t o t h e g r a d u a 1 alteration of the major axes in the bones of 

 the fore arm and of the metapodium respectively, as brought 

 about by reduction, and tend to maintain these proximal and 

 distal axes in the same vertical line. 



3. The initial displacement, however, preceding and independent of reduction, is 

 the e c t a 1 movement o f t h e m e t a p o d i a 1 s , adapting these elements to resist 

 the strain of the "stroke" upon the outer border, as the foot extends downwards and 

 inwards. 



4. Til ihe unreduced isodactyl types, the strain ol" the spreading metapodials 



