130 
of the middle portion of the sharp anterior edge, there is, in 
Phascolonus, a conspicuous twist in the continuity of the bone of 
such a kind as would arise from a similar inflection of both the 
anterior border of the lower end and of the posterior border of 
the upper. From the combined effect of the twist, as atfecting 
the upper end, and of the considerable actual production, 
postero-internally, of the latter it arises that the proximal surface 
is not only of great superficial extent but also asymmetrically 
disposed by reason of its extension in conformity with that of 
the head. The condylar depressions of this surface show a 
similar asymmetry in size as well as in position, and both are 
thrown far back upon it, so that a large area of non-articular 
surface lies in front of them, the latter feature being also present 
in the wombat. The inner condylar depression, considerably 
the larger of the two, is ovoid in shape and deeply concave, the 
depth of the concavity being contributed to by the encroachment 
of its articular surface on the inner and higher of the two antero- 
posteriorly disposed ridges which, together, form the spine of the 
tibia. The surface for the outer condyle is sub-polygonal in 
shape and generally flat except for that portion of it which rises 
on to the outer of the two spinous ridges. In the wombat there 
is less inequality of size between the condylar surfaces and the 
outer is distinctly convex though the inner is concave. 
In Phascolomys the superior tibio-fibular articulation, which is 
nearly flat antero-posteriorly, to a large extent underlies the 
the outer condylar surface, the two articular tracts making an 
acute angle with one another. In the fossil this angle is less 
acute and a distinct degree of fore-and-aft concavity of the 
fibular articular surface is expressed. 
The internal malleolus of the lower end of the fossil tibia is 
very broad antero-posteriorly ; it is not, as in the wombat, pro- 
duced, inferiorly, into a compressed conical process ; and the 
articular surface for the astragalus, instead of extending, as in 
the latter, on to the whole of the external face of the malleolus, 
encroaches upon it but to a very limited extent. It is with this 
latter part that is articulated the inner surface of a low promi- 
nence that will be afterwards described as rising from the inner 
border of the tibial surface of the astragalus. 
A depression on the articular surface of the distal end of the 
tibia, just external to the malleolus, receives the summit of the 
projection itself. 
A distinct triangular facet for the os pyramidalis exists which 
makes an obtuse angle both with the astragalar surface and with 
the plane of the hinder region of the extreme lower part of the 
external surface of the shaft. 
