PEOFESSOB, OWEN ON THE SPECIES OF PHASCOLOMTS. 495 



As in the radius, the shaft of the tibia (PI. LXXIV. figs. 5-7) is compressed (fig. 7), 

 slightly bent (figs. 5, 6), with the concave border rather sharp ; and the shaft gains 

 thickness as it approaches the distal end. 



The proximal end (fig. 8) retains the usual superior extent of articular surface in this 

 bone of the hind limb as compared with its homotype in the fore limb. The surfaces 

 (a, h) adapted to the femoral condyles are partially divided posteriorly by a shallow 

 groove (c), the sides of which, especially the inner one, rise to give attachment to the 

 ' crucial ligaments ;' the chief and lai'ger 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, g), 

 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. G), 

 which includes what in most tibioe 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, 1, 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 (J) 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. LXXIV. fig. 7, k) is unmistakable in -the Wombat. This process (fig. 9, b) arti- 

 culates with the scaphoid or naviculare of the tarsus (PI. LXX. fig. 8, s), whilst the 

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

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



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

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



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