SLOTHS AND THEIR HAIR 315 
Space not longer than the one between the second and 
third. In the two-toed form, on the other hand, the first 
tooth is taller than those behind, and has a bevelled 
instead of a flat grinding surface, while the space dividing 
it from the second much exceeds that between any of 
the others. Again, the front of the upper jaw of the 
two-toed sloth carries a T-shaped bone, corresponding to 
the premaxillae of other mammals, which is totally wanting 
in the other species. The front of the lower jaw of the 
former is also prolonged so as to form a kind of spout, 
of which there is no trace in the latter. In both these 
respects the two-toed sloth comes much nearer to the 
extinct ground-sloths than is the case with its three-clawed 
cousin. 
Again, if the males of the three-toed sloth be examined, 
there will be seen a patch in the middle of the back where, 
owing to the absence of the long coarse external hair, 
the presence of a soft orange and brown under-fur is 
shown. It has been stated that this patch of under-fur 
is made visible by the animals rubbing their backs against 
boughs and wearing off the long hair, but it seems much 
more probable that it is a sexual character. Of this under- 
fur the two-toed sloth has but a very imperfect development. 
Apart from its extremely coarse and brittle nature, the 
most striking peculiarity of the outer hair of the sloths is 
its more or less decidedly green tinge. To see this in 
perfection it is necessary to examine living animals, as it 
tends to fade away more or less completely in skins 
long exposed to the light, leaving the hair of a pale greyish 
brown colour. 
Now green is a very rare colour among mammals, and 
there ought, therefore, to be some special reason for its 
development in the sloths. And, as a matter of fact, the 
