Professor Owen on the Megatherium. 93 
ho existing ground-dwelling Edentate is the fifth digit deprived 
of its ungual phalanx, as in the Megatherium. The bones of the 
fore foot of that extinct animal are thus seen to be modified 
mainly after the type of the Bradypodide. - ? 
The long bones of all the limbs are devoid of medullary cavi- 
ties, as in the sloths. 'The femur lacks the ligamentum teres, as 
in the sloths. The fibula is anchylosed to the tibia at both ends 
in Megataerium, as in Dasypus ; but this is not the case in the 
closely allied extinct Megatherioids called Mylodon, Megalonyz, 
and Scelidotherium, a fact which diminishes the force of the ar- 
gument which Cuvier deduced from the coalesced condition of 
the bones in the Megatherium in favor of its affinities to the ar- 
madillos. The semi-inverted but firm interlocking articulation 
of the hind foot to the Jeg shows the peculiarities of that joint 
in the sloths exaggerated, and departs further from its character- 
istics in other Edentata. In all the existing H'dentata, save the 
sloths, the hind foot is pentadactyle, and four of the toes have a 
ong claw, even in the little arboreal Myrmecophaga didactyla; 
the departure by degradation from the pentadactyle type is a pe- 
culiar characteristic of the sloth tribe in the order. It is carried 
further in the same direction in the great extinct terrestrial sloths. 
n these the mutilation of the foot has commenced on the outer, 
side by the removal of the ungual phalanx from the fifth and 
fourth toes; but this accompanied by modifications, which adapt 
these toes to the important office of support and progression of 
the body on level ground. In the scansorial sloths, the three 
