HISTORY OF THE EDENTATA 605 



ably derived from ancestors which in the Pliocene 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 fGravigrada are comparatively com- 

 mon in all of the fossiliferous formations between the Pampean 

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

 any useful hght upon the development of the various famiUes. 

 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 accoimt of the tgrouiid-slotbs 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 Unes, for 

 hardly any two specimens are alike and therefore the satis- 

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

 with aU this remariiable variabihty, the range of structural 

 differences was not great ; the group was a very homogeneous 

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

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

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

 fMegalonychidaB, which in the South American Pleistocene 

 had dwindled to such insignificant proportions, formed the 

 overwhelming majority of the Santa Cruz fGravigrada; the 

 fMylodontidae were quite rare in comparison and are stiU 

 very incompletely known; while the fMegatheriidae, though 

 probably present, have not been identified beyond aU doubt. 



All of the Santa Cruz fgroimd-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 modem tree-sloths. This was a wonderful difference be- 

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

 involved nearly aU other groups of manunals. So far as the 

 skeleton is concerned, this is known with completeness only 

 for the fMegalonychidae, especially the genus ^Hapalops; but 



