1917] Stock: Structure of the Pes in Mylodon harlani 



285 



the span of the straddle as measured hy the distance between right 

 and left imprints. He considered the possibility that the tracks had 

 been made by a bear or by a ground-sloth. In an addendum to the 

 paper already referred to, LeConte mentions the suggestion of Marsh, 

 that the supposed human imprints were perhaps formed by a ground- 

 sloth, either Mylodon or Morotherium. 35 At that time remains of the 

 latter forms were known to occur in Great Basin deposits of same 

 age as the Carson strata. Marsh''" compared the imprints with the 

 pes of Mylodon and expressed an opinion in favor of their association. 



The objections to the quadrupedal origin of the footprints were 

 stated by LeConte to rest in "the apparent singleness of the tracks, 

 and the absence of claw-marks." The first of these he explained by 

 the observed fact that the imprint of the posterior foot is often super- 

 imposed upon that of the anterior foot, while he attributed the second 

 peculiarity to "the clogging of the feet with mud." 



From the present reconstruction of the pes of Mylodon, it is seen 

 that the third digit did not bear a large claw as supposed by Owen. 

 It follows that the possibility of obtaining a distinct impression of 

 this claw is therefore also reduced. A comparison between the pes of 

 Mylodon harlani from Rancho La Brea and the debatable footprints 

 at Carson City (see figs. 5 and 6) reveals a remarkable similarity in 

 size and general outline. This evidence is then in strong confirmation 

 of the view expressed by LeConte and Marsh. 



The common occurrence of M. harlani in Pleistocene deposits of 

 North America makes it not improbable that the imprints at Carson 

 belong to an individual of this species. The possibility remains, 

 however, that the footprints were made by a ground-sloth other than 

 Mylodon. Megalonyx and Nothrotherium are known to have been 

 present in the Sierran region during the Pleistocene. 



Conclusions 



In the restoration of the pes of Mylodon harlani, based upon 

 material from Rancho La Brea, the second and third phalanges of 

 digit 3 are identified Math the corresponding phalanges of digit 2, 

 manus of M. robustus, as interpreted by Owen. In Owen's recon- 

 struction of M. robustus the second and third phalanges of digit 2, 



35 This genus, as defined by Marsh, has been shown by the writer (Seience, 

 n. s., vol. 39, pp. 761-763, 1914) to be in part synonymous with Mylodon. 



38 Marsh, O. C, On the supposed human footprints recently found in Nevada, 

 Amer. Jour. Sei., ser. 3, vol. 26, pp. 139-140, 1883. 



