67 



proportions corresponding with those of the femur. The proximal end of the tibia 

 (Plate XXIV. fig. 3) presents two distinct and well-marked articular surfaces ; the inner 

 one (?) is concave, the outer one (e) is convex : the extent of these surfaces corresponds 

 with the breadth of the articular part of the outer and inner femoral condyles respect- 

 ively. The back part of the outer facet which bends downward affords an articulation 

 (f) to the fabella. The rough interspace between the articular surfaces is a little con- 

 cave transversely, and convex from before backward ; its breadth equals that of the outer 

 surface : it developes no intercondyloid process for crucial ligaments. 



The fore part of the proximal end of the tibia presents a large irregularly triangular 

 rugged protuberance for the ligamentum patellae; the back part, below the outer con- 

 dyle, developes a smaller but more prominent rugged process, the apex of which over- 

 hangs the upper part of the interosseous space: between this 'post-tibial' process and 

 the rough inner border of the bone there is a deep and wide longitudinal channel 

 inclining a little obliquely to the interosseous space. The shaft of the tibia gradually 

 contracts to its middle, and as gradually expands to its distal end. It is subcompressed 

 from before backward ; is smoother behind than in front : there is a longitudinal 

 channel on each side the back part of the lower end of the tibia, which forms a con- 

 vexity between them. 



The anterior surface is divided by a ridge extending obliquely from the rotular pro- 

 tuberance to the inner malleolus : the surface on the fibular side of this ridge is smoother 

 than the other, which seems to have been wholly given up to musculo-tendinous attach- 

 ments. The inner malleolus (Plate XXIV. fig. 4, m) is a slight expansion below the 

 confluence of the inner and oblique anterior ridges: it does not project below the level 

 of the distal articular surface. This is deeply concave, and is divided into two facets by 

 the deeper hemispherical excavation near its inner side for the reception of the inner 

 protuberance of the astragalus: this excavation (e^ gives to the larger and shallower 

 facet (?) a full crescentic figure. The smooth surface is sometimes continued upon, 

 sometimes interrupted by, a narrow tract from the vertical surface upon the malleolar 

 end of the fibula, which surface is applied to the outer facet of the astragalus. There 

 is a small orifice for a medullary artery at the middle of the back part of the tibia, but 

 it does not open into any medullary cavity : the bone is cancellous throughout. 



The fibula is thickest at its upper end, where it has a trihedral form ; the outer surface 

 is convex and rough, the inner and hinder surfaces are concave and smooth, meeting at 

 a sharp interosseous border directed obliquely backward : this border is slightly thickened 

 and produced at its middle part, where the shaft of the fibula is compressed: it 

 augments in thickness and resumes its trihedral shape as it descends, and terminates 

 in a moderately produced outer malleolus (/) with a very rugged surface, except where it 

 articulates with the astragalus. 



In the Mylodon, as well as in the Megalonyx and Scelidotherium, the tibia and fibula 

 continue separate, — a fact affecting the value of the evidence which Cuvier deduced from 

 their anchylosed condition in the Megatherium in favour of its affinities to the Arma- 



