286 University of California Publications in Geology [Vol. 10 



manus, have undoubtedly been interchanged with the corresponding 

 phalanges of digit 3, pes. 



A comparison of the pes of Mylodon harlani with that of Scelido- 

 therium indicates certain resemblances between the two genera, which 

 support their family relationship already suggested by the lobate 

 character of teeth. In contrast to Mylodon, in which the first or 

 inner digit is lost, the pes of Scelidotherium is markedly more special- 

 ized in the additional loss of a functional second digit. The latter 

 genus differs further from Mylodon in the large size of the third 

 ungual, agreeing in this character with other members of the Gravi- 

 grada, as, for example, Megalonyx and Megatherium. 



The rotation of the pes toward the outer side has progressed much 

 further in Mylodon harlani than in any of the Miocene ground-sloths 

 from the Santa Cruz beds. This process is accompanied by a number 

 of modifications adapted to a highly specialized type of locomotion. 



With allowance for the differences in interpretation, the stage of 

 evolution of the pes in M. harlani is near to that of M. robustus. 



In size and general outline the pes of Mylodon harlani is remark- 

 ably similar to the supposed human footprints occurring at Carson, 

 Nevada. 



Transmitted December 15, 1916. 



