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 (Megalotherium) 
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 
