PROFESSOR OWEN ON THE FOSSIL MAMMALS OF AUSTRALIA. 
565 
extends for an inch or two down the shaft. Beneath the outer tuberosity is a rough 
shallow cavity (ib. fig. 3, Jc), and a similar but smaller one (ib. 1) impresses the shaft a 
little way below the fibular articular surface. An internal tuberosity (figs. 2 & 4, m) is 
feebly marked below the contracted inner end of the inner articular surface. 
The shaft of the tibia rapidly contracts to a transverse diameter of inches at the 
middle third of its extent, where it is trihedral, with the angles rounded oft'. It ap- 
pears to be twisted with the inner malleolus turned forward ; but this is very feebly 
marked, not projecting below the distal articular surface. At the outer and back part 
of the lower half of the shaft is a rough longitudinal prominence (fig. 3, n), 4 inches by 
1 inch, seemingly for ligamentous attachment of the corresponding part of the shaft of 
the fibula. At the inner and back part of the shaft a low narrow fibrous ridge runs 
parallel with the inner border of the fibular ridge, defining therewith an oblique shallow 
canal, 9 lines in width. 
A slightly raised border of bone (figs. 2, 3, p ), from 1 to 2 inches distant from the 
lower articulation, seems to indicate the original line of junction of the epiphysis. 
Malleoli cannot be predicated of the distal end of this tibia (ib. fig. 5). At the inner peri- 
phery of the articular surface, instead of a prominence there is a notch (ib. q ), from which 
a groove 1| inch long and 5 inches wide extends outward and forward into the joint ; 
the rough convex border of the articulation external to this, corresponding in position 
to the fore part of the upper end of the tibia, appears to represent an internal malleolus. 
On each side the entering groove ( q ) the distal articular surface is slightly convex ; in the 
rest of its extent it is nearly fiat ; its form is oblong, with the long axis at right angles to 
that of the upper articular surface, i. e. from before backward instead of from side to 
side. 
In a portion of the shaft of a tibia, obtained by Sir Thomas Mitchell from the bed 
of the Condamine Biver, the upper part of the ridge between the outer and hinder 
surfaces shows the orifice of a medullary arterial canal, which expands as it slightly 
descends. No medullary cavity, however, is shown in this fragment. The compact part 
of the wall of the shaft is half an inch thick, and a moderately close cancellous structure 
extends inward to the centre of the shaft. 
A subtrihedral portion of bone, including the distal end and accompanying the 
above portion of tibia, I believe to be part of the fibula ; it is 7 inches in circumference. 
The centre of the shaft is occupied by a close cancellous texture. The articular extre- 
mity is much abraded ; a trace of the epiphysial suture remains ; and I find that this is 
long conspicuous in the fibula of the Wombat. I have given what I conjecture to be the 
proportions of the fibula in my restoration of the skeleton of Diqtrotodon, Plate L. 
In the singular form of the tibia of Diprotodon are presented Marsupial characters 
exclusively. “ The outer articular surface is continuous with that of the head of the 
fibula”*, as in the Wombat and Koala ; “ the shaft of the tibia is twisted as in Opossums, 
Dasyures, Phalangers, and Petaurists, as well as in the Koala and Wombat”*. 
. . * Osteology of the Marsupialia, loc. cit p. 405. 
