PEOFESSOE OWEN ON THE aVIEOATHEBIEM. 
814 
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 ndge is smoother 
than the other, which seems to have been wholly given up to musculo-tendinous attach- 
ments. The inner malleolus (Plate XXXIX. 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 mner 
protuberance of the astragalus : this excavation (e) gives to the larger and shallower 
facet (i) a full crescentic figure. The smooth surface is sometimes continued upon, 
sometimes interrupted by, a narrow tract from the vertical surface upon the maHeolai- 
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 trbia, 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 irrterosseous border directed obliquely backward : this border is shghtp 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 terTninates 
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 trbra and fibula 
continue separate, a fact affecting the value of the evidence which Cuviee deduced ffom 
their- anchylosed condition in the Megatherium in favour of its affinities to the Arma- 
dillos, to which this structure is peculiar amongst existing Mammals. Birt, smce rt rs 
known only in the order Bruta, it forms an interesting additional proof of the essentrcff 
relations of the huge extinct airimal under description to that now anomalous group of 
Mammals. The tibia of the Mylodon is proportionally shorter and thicker than in the 
Megatherium: the outer articular surface at the upper end is of a subchcular- form 
and slightly concave : in the Megalonyx it is corrvex, but in a less degree than in the 
Megatherium. The lower articular surface in both Mylodon, Scehdotheriirm, and Mega- 
lonyx, presents the additional facet for the fibula. The hemispheric excavation on the 
inner side of the distal articulation is relatively larger in the Mylodon than in the Mega- 
therium. This excavation, with the concomitant protuberance of the astragalus, is pecu- 
liar to the great extinct Sloth-like quadrupeds ; in which so secure an interlockmg of 
the foot with the leg bespeaks some habits peculiar to them, corrnected w ith thereqriire- 
ment of unusual resistance in the foot to the forces acting upon it from the leg arrd 
thigh. 
In the series of existing animals Man presents the plantigrade foot in which the 
weight of the body presses most nearly upon the crown of the tarsal arch ; but, in the 
Megatherium, owing to the length of the heel and the shortness of the toes, the leg 
transmits the superincumbent weight nearly upon the middle of the foot. So singulaily 
