SOME EXTINCT ARGENTINE MAMMALS tor 
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 
