HISTORY OF THE EDENTATA 605 



ably derived from ancestors which in the PHocene migrated 

 from Central America. Aside from certain remarkable pecu- 

 liarities of the teeth, this animal was more primitive, as well as 

 smaller, than any other of the Pleistocene genera. 



Although remains of jGravigrada are comparatively com- 

 mon in all of the fossihferous formations between the Pampean 

 and the Santa Cruz, the material is too imperfect to throw 

 any useful light upon the development of the various families. 

 From the Santa Cruz beds, on the other hand, a great wealth of 

 specimens has been obtained, and it is possible to give some 

 fairly adequate account of the tground-sloths of that time. 

 These animals were then extremely abundant individually and 

 of extraordinary variety ; evidently, they were in a state of 

 rapid expansion and divergent evolution along many lines, for 

 hardly any two specimens are alike and therefore the satis- 

 factory discrimination of species is well-nigh impossible. Yet, 

 with all this remarkable variability, the range of structural 

 differences was not great ; the group was a very homogeneous 

 and natural one, and separation into families was not obvious. 

 Two of the three families were, however, unequivocally pres- 

 ent in this fauna and the third somewhat doubtfully so. The 

 fMegalonychidse, which in the South American Pleistocene 

 had dwindled to such insignificant proportions, formed the 

 overwhelming majority of the Santa Cruz fGravigrada ; the 

 fMylodontidse were quite rare in comparison and are still 

 very incompletely known ; while the fMegatheriidae, though 

 probably present, have not been identified beyond all doubt. 



All of the Santa Cruz fground-sloths were small animals, 

 the largest not approximating the smallest Pleistocene species, 

 those of Cuba excepted, and many of them were no larger than 

 the modern tree-sloths. This was a wonderful difference be- 

 tween the Santa Cruz and the Pampean, but a difference which 

 involved nearly all other groups of mammals. So far as the 

 skeleton is concerned, this is known with completeness only 

 for the jMegalonychidae, especially the genus ^Hapalops; but 



