SOME EXTINCT ARGENTINE MAMMALS 101 



and grow continuously throughout the life of their owner, 

 this transversely ridged structure is likewise permanent. 

 To contain such enormous teeth, the lower jaw is remark- 

 ably deepened in the middle of its length, where it descends 

 suddenly. A long median channel, extending between and 

 in front of the anterior teeth, is evidently for the reception 

 of a large and fleshy tongue, which from its size was 

 probable extensile like that of the giraffe. 



If we had only the megalothere to deal with, there 

 might be some hesitation, judging from the skull and 

 teeth (which in the group are the only portions of the 

 skeleton showing sloth-like affinities) in regarding the 

 group of animals to which it belongs as closely allied to 

 the sloths. Fortunately, however, the same Pleistocene 

 deposits of Buenos Aires (to say nothing of the caverns 

 of Minas Geraes, in Brazil) have yielded remains of other 

 and somewhat smaller ground-sloths, known as mylodons, 

 which effectually bridge, in these respects, the gap between 

 the megalothere and the sloths. In these animals the teeth 

 are either cylindrical or triangular in section ; and from 

 having a harder external coat, wear in the same cup- 

 shaped manner as those of the latter. Moreover, in 

 some mylodons the front pair of teeth in each jaw have 

 the elongated tusk-like form and oblique wear character- 

 ising those of the two-toed sloth, while in others they 

 resemble the hinder teeth, as in the three-toed sloth. 

 We thus have an exact parallelism in this respect among 

 the mylodons to the two genera of sloths; and as their 

 skulls in their more rounded and shorter form, and the 

 absence of a descending expansion in the middle of the 

 lower jaw, are likewise more sloth-like than is the skull 

 of the megalothere, we can have no hesitation in re- 

 garding the ground-sloths, so far as cranial characters 



