368 



' crucial ligaments ;' the chief and larger division (a) for the inner condyle of the femur, 

 is more concave than usual, especially in Phascolomys platyrhinus, recalling the form 

 of the humeral articular surface of the radius. The rotular tuberosity (figs. 5, 8, d, y), 

 homotypal with the bicipital one of the radius, is lower down and more remote from 

 the proximal articular surface than usual. The fore part of the tibia continued down 

 from this tuberosity soon contracts to a ridge, which near the middle of the shaft 

 projects from the anterior contour and inclines slightly towards the fibula (figs. 5, 6, h). 

 The inner side of the shaft (fig. 5) is broad, smooth, almost flat. The outer side (fig. 6), 

 which includes what in most tibiae is the hinder side, is sinuous at its proximal half and 

 angularly convex across at its distal third (i). The concavity between the back parts of 

 the proximal articular surfaces, answering to the 'popliteal notch' of Anthropotomy, 

 expands, as it descends, into a longitudinal concavity, which merges into the inner side 

 of the bone by the ridge-like backward projection of its inner border, and the low 

 development and speedy suppression of its outer one (fig. 6, I, i). This border is con- 

 tinued lower and becomes more ridge-like in Phascolomys platyrhinus than in Phase, 

 latifrons, marking out more definitely a hinder facet of the tibial shaft, although 

 characteristically narrow, and giving a more concave or canaliculate character to the 

 fore part of the outer surface of the shaft. A hinder surface of the tibial shaft can 

 only be defined in Phascolomys latifrons at the thicker distal half. The general 

 bend, concave backward, of the tibia is greater in Phascolomys latifrons than in 

 Ph. platyrhinus. 



The modification of the distal articular surface of the tibia (fig. 9) resembles that of 

 the radius. The facet for the proximal surface of the astragalus (a) is continued upon 

 the inner malleolus (b) at a more marked angle than the homotypal surface is continued 

 upon the 'styloid process' of the radius (PI. LXXII. fig. 5, e) ; but the homotypy of 

 the part so called in the radius with the part called 'internal malleolus' in the tibia 

 (PI. CIII. fig. 7, k) is unmistakable in the Wombat. This process (fig. 9, b) articulates 

 with the scaphoid or naviculars of the tarsus (PI. XCIX. fig. 8, s), whilst the horizontal 

 facet (fig. 9, a) articulates with the astragalus as its homotype in the forearm articulates 

 with the ' lunar' part of the scapho-lunar bone. 



The fibula in Phascolomys (PI. CIII. figs. 10, 10', 11) presents the rare ulnar character 

 of proximal expansion (a) and olecranal leverage (b) ; only that the terminal summit of 

 the lever retains its individuality and articular union with the basal part of the uprising 

 process. 



The proximal articular surface of the fibula has one facet (fig. 11, d) for the outer 

 condyle of the femur, and a rather more concave surface (e) for the contiguous lateral 

 facet or the proximal end of the tibia (fig. 6, a). The articular surface (figs. 10, 11, a) 

 for the ' fabella' (ib. ib. b) is subcircular and almost flat. 



The proximal half of the fibula is triedral ; the outer facet (fig. 10, /) between 

 the homotype of the olecranon (a) and the antero-external ridge (g) is deeply concave, 



