SOME EXTINCT ARGENTINE MAMMALS 105 



habits, we are in a position to realise the appropriate 

 nature of the term " ground-sloths " by which they are 

 designated. These creatures may, in fact, be briefly 

 described as edentates with a skull, teeth, and shoulder- 

 girdle very similar to those of the sloths ; while as regards 

 their backbone and feet they come very close to the ant- 

 eaters, although in the later and more gigantic forms the 

 specialisation characterising the fore-feet of the latter has 

 been extended to the hinder pair. 



Turning to the question of the mutual relationships and 

 phylogeny of the three groups of edentates discussed in 

 the course of the foregoing paragraphs, we shall have 

 little hesitation in regarding the pigmy ground-sloths, 

 which are the earliest known representatives of the group, 

 as the direct ancestors of the gigantic megalothere. A 

 modification in the structure of the teeth would equally 

 well permit of their having likewise been the ancestors 

 of the mylodons, which, as we have seen, possess sloth- 

 like teeth. This, however, will not permit us to regard 

 the mylodons as having been the forerunners of the sloths, 

 seeing that the latter have a less specialised type of 

 hind-foot ; and we must accordingly regard the sloths as 

 a side branch derived from the pigmy ground-sloths or 

 some nearly allied forms after the acquisition of cylindrical 

 teeth, but before the hind-foot had acquired the specialisation 

 characterising the mylodons and megalotheres. Hence 

 the curious structural similarity between the front teeth 

 of some of the mylodons and the two-toed sloth must be 

 another instance of that parallelism in development to 

 which reference has so often been made. 



With regard to the ant-eaters, we have already seen 

 that the fore-foot of these animals resembles that of the 

 pigmy ground-sloths in that the terminal joints of the 



