3o6 GARY. [Vol. VII. 



The key to structure is use, for which in this study the work 

 Animal Locomotion by Harrison Allen has been taken as 

 authority. According to this author, whose statements in turn 

 are based on photographic studies, the following seems to be 

 true of the gait of all terrestrial mammals with a broad foot. 

 The foot while off ground is carried forward in partial prona- 

 tion, and strikes the ground by its outer border. This it does 

 with ■ the limb straight, and directed well forward at an angle 

 depending on the speed of the animal and the weight of its 

 head and shoulders. The limb as it strikes arrests the down- 

 ward plunge of the body — then it acts as a lever of the third 

 class to bring the body forward. When the limb is vertical, the 

 foot is planted squarely on the ground ; as the perpendicular is 

 passed, the foot rolls on to its inner border. The outer toes thus 

 become free, and they are successively flexed. With a straight 

 thrust, and from the inner border of the foot, the limb leaves 

 the ground, its segments during early recover being flexed on 

 one another to clear the ground and to offer less resistance to 

 the air. 



This much was true in all probability for the forms under 

 consideration. It is evident that weight is borne by the limb 

 in full extension. Pressure therefore occurs chiefly at the 

 anterior face of the foot, while the ligaments which bind the 

 carpal rows to one another and to the bones above and below 

 are needed, as they are placed, at the posterior face. 



Turning to the foot of Palaeosyops with the facts of its use 

 in view, the following principles of its structure are derived : — 



(i) Direct thrust of the metacarpals is distributed by slant- 

 ing surfaces from each side across the foot, and is met by the 

 curve of the cubito-carpal joint. 



(2) The swinging strike of the foot and lateral thrusts from 

 each side are met in the same way, also by interlocking surfaces 

 which take little or none of the direct thrust of the digits. 

 Such surfaces are those between metacarpals IV and V, IV-III, 

 III-II, and Il-magnum. 



(3) Torsions are taken up by these surfaces which are largest 

 at their anterior and posterior ends ; also by backgrowths of the 

 carpals. Projections of scaphoid and lunar centre down from 

 each side on that of the magnum. 



(4) Differentiation in the two sides of the foot. The limb 



