270 University of California Publications in Geology [Vol.10 



The external cuneiform does not differ essentially from the corre- 

 sponding element described by Owen, but is smaller than that shown 

 in the pes of M. robustus. An articular facet for the cuboid has not 

 been observed on any of the specimens from Rancho La Brea, although 

 some do show a facet on the inner side for the middle cuneiform. 

 Owen 4 describes the latter element as follows : 



The internal of the two cuneiform bones is about half the size of the external 

 one, and of nearly the same antero posterior diameter, but more compressed 

 laterally, and of somewhat less depth: its convex base is uppermost; its obtuse 

 apex downwards: the inner and outer sides are nearly flat, rough, without 

 trace of smooth articular surface, which is limited to the anterior and posterior 

 extremities of the bone. The posterior surface is elliptical, slightly concave, 

 adapted to the distinct circumscribed articular surface on the naviculars: the 

 anterior articular surface is nearly circular, and very slightly convex. There is 

 no articular surface upon the tibial or inner side of this cuneiform bone, nor any 

 indication of a third or internal cuneiform on the os naviculare; from which it 

 is to be concluded that the internal cuneiform bone and first toe, or hallux, were 

 altogether wanting in the Mylodon, and that the mutilation by which its hind- 

 foot is reduced to the tetradaetyl type has commenced, according to the ordinary 

 law, from the inner side. 



In Mylodon harlarri the middle cuneiform is compressed laterally 

 and is roughly wedge-shaped (figs. 3b and 5) with the anteroposterior 

 diameter of the rounded and narrow dorsal surface twice as great as 

 the corresponding diameter ventrally. The depth of the cuneiform 

 may be greater or less than the greatest anteroposterior diameter. The 

 ventral surface is flattened and is wider transversely than the dorsal 

 surface. The outer surface is flatter than the inner and, in all the 

 specimens examined, bears along the proximal margin a facet for the 

 external cuneiform. There is no indication of a facet along the proxi- 

 mal margin of the inner side for an internal cuneiform. The proximal 

 articulating surface for the navicular is rectangular in shape and is 

 slightly concave. At the distal end the articulating surface for the 

 metatarsal is placed oblique to the vertical axis of the cuneiform. It 

 is of greater dorsoventral diameter than the proximal articulation and 

 is more deeply concave in that direction. The surface narrows dor- 

 sally and in most specimens is constricted at about the middle of the 

 internal side. The middle cuneiform is noticeably larger than in 

 M. robustus. 



Metatarsus. — The second metatarsal in M. robustus is described by 

 Owen 5 as follows : 



4 Owen, op. ext., p. 121. 



5 Owen, op. cit., p. 121. 



