A NEW MYLODON. 339 



M. rohuslus. The distal facets are similar in both species. The right-hand tibia 

 is nearly 20 nun. shorter than the other, and a similar discrepancy occurs in the 

 fibulae. Leidy (1889, p. 35) gives 10 inches or 233 mm. as the extreme length 

 of the tibia of M. harlani. In both it is apparently longer than in M. robustus. 



The fibula is of the same general form as in the South American Mylodon 

 with an obliquely truncate articular surface internally, and a thickened external 

 ridge bearing a small facet for articulation with a sesamoid. The roughened 

 portion at the lower half of the shaft is, however, much less apparent, and the 

 shaft itself is rather smooth throughout. The arrangement of the distal facets 

 is quite different. Of M. robustus Owen (1842, p. 116) writes: — " The inner sur- 

 face of the distal expansion presents a concavity, two fiat synovial articular 

 surfaces, and an intermediate rough ligamentous tract. The upperpart of the 

 concavity is very irregular; its lower part is formed by a flat obUque articular 

 surface. * * * A narrow, slightly concave, transverse tract divides the upper from 

 the lower articular surface; this is of less extent than the one above; its plane 

 is vertical, looking directly inwards, and is adapted to the flat surface on the outer 

 side of the astragalus." In the specimen studied the two facets (Plate 4, fig. 21) 

 are present as described, the upper one with its plane slightly oblique, but instead 

 of a "transverse tract" separating the two, their boundaries are contiguous at 

 the angle where the two planes intersect. In this respect the Nebraska specimen 

 is similar to Paramylodon as described by Brown so that the character cannot be 

 ascribed generic value. The astragalar facet occupies about one half the width 

 of the distal end of the bone, and is vertically elongate. The Mylodon fibula 

 described and figured by Williston (1895, p. 175) is larger than either of those 

 studied, and has the two distal facets separated as in M. robustus. It was ob- 

 tained at Seneca, Kansas, from a well, 50 feet below the surface in alluvium. 

 WiUiston refers it provisionally to M. harlani, at the same time calling attention 

 to its apparently larger dimensions, as deduced from Leidy's measurements 

 of tibiae. Its greatest length was 290 mm. Of the two fibulae of the specimen 

 here described that of the left-hand side is the longer, 265 mm. ; that of the right- 

 hand side is less, 234 mm. These latter measurements accord more nearly with 

 what may be assumed, from the tibiae, to be the length of fibula in M. harlani. 

 As suggested by Williston, his specunen may represent a different species, but 

 until undoubted fibulae of M. harlani are found, this point cannot be settled. 



The bones of the hind foot seem to be essentially similar to those of M. 

 robustus, as described and figured by Owen. The peculiarly shaped astragalus 

 is smaller than that of Paramylodon, 140 mm. in greatest width, and 111 mm. 



