188 COPE. [Vot. III. 
The mechanical causes of these structures appear to be as 
follows. The strained incurvature of the foot has rotated the 
astragalus outwards on the tibia, so that the latter comes to 
articulate with its inner side, which has become continuous with 
the superior face by pressure. The stage intermediate between 
the normal tibio-astragalar articulation, and that of the sloth, is 
seen in Megatherium and Mylodon, where the inner part of the 
trochlea is reduced to a rudiment. In Bradypus it has dis- 
appeared. The pressure of the tibia has had the effect to throw 
the external face of the astragalus against the distal fibular 
articulation. In the sloths the articular ligaments connecting 
the fibular with the external face of the astragalus have been 
continuously strained, and the result has been the elongation of 
their more or less circular line of insertion, producing a promi- 
nent border which embraces the extremity of the fibula like a 
cup. And continual rotation has given the extremity of the 
fibula a conical form (Fig. 25 & a). 
The movable articulation of the astragalus with the navicular 
bone is seen in Megalonyx, Bradypus, and to a less degree in 
Myrmecophaga. The foot rotates inwards on the long axis of 
the astragalus, which has around head. This structure may 
be accounted for by the continued effort to perform the move- 
ment which it permits. It is to be noted that on this articula- 
tion the articular surface of the head of the astragalus presents 
a fossa, or a pit, into which a corresponding prejection of the 
navicular fits. The mechanical cause of this structure is to 
be accounted for by the ligamentous strain accompanying the 
vigorous use of the claws, which these Edentata exhibit in 
their habits. But why a rim border should be drawn from the 
astragalus rather than from the navicular, I am not able to 
explain. The calcaneum is excavated for the head of the astra- 
galus, and the sustentaculum is convex; both arrangements 
permitting the rotation of the foot plus the calcaneum on the 
astragalus. 
f. The Tongue-and-Groove Joints. These are the following : 
I. Looking downwards. 
1. The distal humeral in Artiodactyla and Quadrumana. 
2. The distal radial. 
3. The distal tibial. 
4 and 5. The distal metapodial. 
