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University of California Publications in Geology [Vol. 10 



ually accumulated and has received considerable addition within the 

 past few years. Comparatively little is at present known, however, 

 of the foot-structure in these forms. Fortunately the excellent and 

 very abundant material of Mylodon harlani from Rancha La Brea, 

 now in the palaeontological collections of the University of California 

 and the Museum of History, Science and Art of Los Angeles, permits 

 as complete an understanding of the manus and pes in the North 

 American species as in the South American M. robustus. 



During the course of an investigation of the structure of the pes 

 in Mylodon harlani, certain differences between this species and M. 

 robustus have presented themselves which apparently are not of specific 

 character, but are due rather to a new interpretation of the elements 

 involved in the former species. As there is reason for offering an inter- 

 pretation of the foot-structure differing in a few particulars from that 

 of Owen, the writer has attempted to present a reconstruction of the 

 pes of Mylodon based upon the assembled foot material of M. harlani 

 from Ranch o La Brea. In this study Owen's description and figures 

 of the pes of M. robustus have been particularly helpful. 



Structure op Pes 



In number of elements present the pes of Mylodon harlani is similar 

 to that of M. robustus. It is worthy of note that in both species digit 

 1 and the internal cuneiform have totally disappeared. The presence 

 of claws on digits 2 and 3 and the amount of reduction which has 

 occurred in the fourth and fifth digits are as characteristic of the 

 North American as of the South American species. Two distinctive 

 differences, among others existing between the two species (see figs. 

 1 and 2), are at once noticeable in a comparative study of the pes: 



( 1 ) The relative size of the middle cuneiform and metatarsal 2 ; 

 and (2) the size of the second and third phalanges of the median digit. 

 These differences will be considered in greater detail below. 



Tarsus. — The posterior border of the tuberosity of the calcaneum 

 is more rounding than in M. robustus. The tendinal notch on the 

 external side is larger than in the South American species and opens 

 rather widely, while along the inner border the calcaneum, at approxi- 

 mately the middle, is much thicker dorsoventrally. The astragalus 

 differs from that of M. robustus in the sharper angle between the two 

 confluent articulating surfaces for the tibia. Lull 3 has described in 



s Lull, B. S., A Pleistocene ground sloth, Mylodon harlani, from Eock Creek, 

 Texas, Amer. Jour. Sei., vol. 39, pp. 375-378, 1915. 



