SOME EXTINCT ARGENTINE MAMMALS 103 



prismatic teeth like those of the megalothere, and not the 

 cylindrical or triangular ones of the mylodons and scelido- 

 theres ; thus apparently indicating that the former type 

 of tooth is the oldest. The contrast between the pigmy 

 ground-sloth and the giant ground-sloth {Megalotheriuni) 

 is, however, most remarkable. The total length of the 

 skeleton of the former was only about three feet, while 

 its skull was less than six inches, whereas that of the latter 

 was over a couple of feet in length. Then, again, the 

 whole series of five upper teeth occupy in the pigmy 

 ground-sloth a space of less than an inch and a half, 

 or less than the diameter of a single tooth of its 

 gigantic relative. That such a diminutive creature, if as 

 naked and undefended as its huge cousin appears to have 

 been, needed some special protection, is evident ; and it 

 is the need of such defence from attack that has led me 

 to suggest that the creature may have been fossorial in its 

 habits. 



Leaving for a moment the mutual relationships and 

 affinities of all these different animals, a glance may be 

 directed at the skeleton of the body and limbs of the 

 ground-sloths. In the first place this differs from that 

 of the sloths in the shortness and extreme massiveness of 

 the limbs ; and especially in the extraordinary stoutness 

 and width of the bones of the hind-leg and haunches. 

 In the general form of the scapula or blade-bone, and 

 more especially in the presence of a complete pair of 

 clavicles or collar-bones, the ground-sloths resemble the 

 sloths and differ from the ant-eaters ; the clavicles of the 

 latter being rudimentary. The skeleton of the fore-foot 

 is, however, essentially that of an ant-eater, the inner toe 

 being rudimentary, the next three, and more especially 

 the middle one, enormously enlarged, and furnished 



