Mylodon harlani, from Rock Creek, Texas. 373 



The proximal articular area forms a broad platform supported 

 from beneath anteriorly by the shaft, but overhanging it though 

 strongly buttressed on the inner side. Posteriorly and exter- 

 nally the overhang is very pronounced, especially beneath the 

 articulation of the external femoral condyle. Here, however, 

 a facet, sub-triangular in outline, marks the point of articula- 

 tion of the h'bula which thus lent its support to withstand 

 the crushing weight transmitted through the thigh. This 

 facet looks outward and downward at an angle of about 45°. 

 The cnemial crest arises somewhat broadly on the outer for- 

 ward angle of the proximal end of the tibia and, narrowing 

 to an acute angle, fades into the surface of the shaft a little 

 below its mid-length. Distally the tibia dilates, though not so 

 widely as above, the greatest distal width being a little more 

 than half the length of the bone in both specimens. The shaft 

 is an elongated oval in transverse section at the point of its 

 least circumference. 



Proximally the articular surface bears two unequal facets, a 

 greater concave one to receive the inner femoral condyle, and 

 a lesser flat area for the external one ; both are sub-oval in 

 outline. Distally the articular surface is very peculiar, as it is 

 the complement of the oddly shaped astragalus. There are 

 three very distinct areas, although the synovial surface is con- 

 fluent. The outermost one is small, flat, and almost semi-circular, 

 facing outward and downward at an angle of about 45° instead 

 of being almost horizontal as in Owen's description of M. 

 robust us. As he says (p. 116), " it forms the lower surface of the 

 outer distal protuberance of thebone,and rests upon a correspond- 

 ing surface at the lower part of that excavation of the fibula, 

 which receives the said protuberance." This surface lies almost 

 directly beneath the fibular facet at the proximal end of the 

 tibia and the two surfaces are approximately but not quite par- 

 allel. The remainder of the distal end of the tibia articulates 

 exclusively with the astragalus and consists of two surfaces, 

 that on the inner side being an ovoid depression which cuts 

 deeply into the remaining area, the line of union describ- 

 ing an arc of about 180°, although the curve is not the seg- 

 ment of a true circle, being somewhat flattened on the outer 

 side. The larger articular surface thus formed is irregularly 

 crescentic, concave in its outer moiety, and flattened toward 

 the inner side of the bone. The tibial shaft bears heavy rough- 

 enings for muscular attachment and the entire bone gives evi- 

 dence of the enormous muscular power of the foot and leg. 



I have before me two casts of this element from the Per- 

 kins specimen of M. harlani {vide sitpra p. 364). Neither is 

 perfect, especially at the proximal end, which gives the bone a 

 totally different aspect. They seem to have been more slender 



