282 University of California Publications in Geology [Vol. 10 



Mylodon, der ausserste zugleieh noeh den calcaneus beriihrt." 20 In 

 M. harlani the fifth metatarsal agrees more closely in shape with 

 SceUdotheriwm than with M. robustus, and does not touch the cal- 

 caneum as in the latter species. 



It is now generally conceded that Scelidotherium is a member of 

 the Mylodontidae, a fact which is especially shown by the lobate 

 character of the teeth. The present observations on the pes indicate 

 that, with due allowance for variations in the individual elements of 

 both genera, certain resemblances between Mylodon and Scelidothe- 

 rium, are closer than was supposed by earlier authors. Associated 

 with these characters are modifications in Scelidotherium which remind 

 one strongly of other Gravigrada. 



Modification and Specialization op Pes 



The twisting of the pes in Mylodon harlani, which brings the 

 weight of the body to bear mainly upon the outer side of the foot, has 

 profoundly affected the morphological structure. It has brought 

 about a number of adaptations suited to a highly specialized type of 

 locomotion. Several of the peculiar modifications to be noted in the 

 pes of Mylodon harlani, some of which are of family, others of generic, 

 distinction, may be listed as follows: 



( 1 ) Large and heavy calcaneum ; 



( 2 ) Distinct angle between the confluent outer and inner divisions 

 of the tibial articulating surface of the astragalus; 



(3) Broadening and flattening, especially of the outer, posterior 

 portion, of metatarsal 5 ; 



(4) Reduction suffered by the fourth and fifth digits; 



(5) Abbreviation of digit 3, with a much smaller ungual than in 

 the corresponding digit of the manus ; 



(6) Abbreviation of the second digit and metatarsal, with which 

 is associated a much smaller ungual than in the corresponding digit 

 of the manus ; 



(7) Loss of digit 1 and the internal cuneiform. 



One of the important modifications of the pes, lending itself to 

 a comparative study, is that shown by the astragalus. In a de- 

 scription of the ground-sloths from the Santa Cruz Miocene deposits 

 of Patagonia Scott has referred to Nematherium an astragalus which 



-'■> Burmeister, H., op. cit., p. 380. 



