137 



prolongation in the Unau. In this species the middle and external cuneiform 

 bones are distinct, and each supports completely developed toes ; but in the 

 Ai the cuneiform bones are confluent with the metatarsals of these toes, 

 and also with the navicular and cuboid bones. Both species of Sloth have 

 three fully developed toes, with long claws, corresponding with the second, 

 third and fourth in the pentadactylous foot : the fifth toe is represented by a 

 rudiment of the metatarsal, which is a little more developed in the Unau than 

 in the Ai. 



We have seen that in the Megatherioids the mutilation of the foot has com- 

 menced on the outer side by the removal of the ungual phalanx from the fifth and 

 fourth toes ; and that the Sloths alone, amongst the existing Edentate animals, 

 offer any real or well-marked afiinity to the Megatherioids, in the degree in 

 which they manifest this characteristic modification of the hind-foot ; but the 

 difierences in this respect between the existing and extinct Phyllophagous Eden- 

 tata are such as might be expected in creatures so different in size, in habitat, 

 and in mode of progression. 



The degradation of structure on the outer side of the foot in the Megatherioids 

 affects two toes equally, but is accompanied by modifications which adapt these 

 toes to the important office of the support and progression of the body on 

 plane ground : in the scansorial Sloths the three middle digits being equally 

 developed for prehensile offices, one toe on the outer and one on the inner side 

 of the foot are reduced to their metatarsal basis. In the Megatherioids the 

 mutilation of the foot on the inner side is carried to a greater extent ; the in- 

 nermost or first toe, with its cuneiform bone, is wholly removed : in the Mega- 

 therium the second toe is represented, like the first in the Sloths, by its cunei- 

 form bone, or at most by a rudimental metatarsal early and very completely con- 

 fluent therewith : in the Mylodon the second toe is fully developed, but of small 

 size. In both the extinct species only the third toe can be compared, by its 

 size, and especially by that of the claw which it supports, with the condition of 

 the three perfect hind-toes in the climbing Sloths. 



Thus the great extinct Sloths present an unequivocal exception to the rule that 

 Unguiculate quadrupeds have pentadactylous feet ; since not even a vestige of 

 the bones of the inner toe, or of the tarsal bone immediately supporting it, 

 exists. 



