A NEW MYLODON. 339 
M.robustus. The distal facets are similar in both species. The right-hand tibia 
is nearly 20 mm. 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. harlanz. 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 flat 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 oblique 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 
b) 
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. 
Williston 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, 234mm. 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 specimen 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. 
