98 MOSTLY MAMMALS 
To justify myself, and at the same time to enable my 
readers properly to understand the structure of these 
strange extinct edentates, it is necessary to enter into a 
short dissertation on the subject of sloths, and likewise of 
their distant cousins the ant-eaters. 
The external form and long shaggy hair of the sloths are 
too well known to require description, and I pass on to 
draw attention to certain peculiarities in regard to their 
skeletons and teeth which will aid in explaining the reason 
for the term ground-sloths. In the first place, then, sloths, 
which are comparatively small animals, are characterised 
by their peculiarly short and rounded heads, of an almost 
spherical form. If the skull of one of these animals be 
examined, a total absence of front teeth will be noticed ; 
while the cheek-teeth comprise five pairs in the upper 
and four in the lower jaw. 
As already stated, the teeth in all edentates are devoid 
of the enamel so characteristic of those of other mammals; 
and in the sloths they form short cylinders, of which 
the outer layer is harder than the central core, in con- 
sequence of which their grinding surfaces become slightly 
cup-shaped. In the three-toed sloths (Bradypus) the whole 
of the teeth are of this extremely simple type; but in 
their two-toed cousins (Cholaepus) the first pair in each jaw 
are Jonger than either of the others, and modified into a 
somewhat tusk-like form, the upper ones wearing against 
the front of the lower ones so as to produce by mutual 
attrition an oblique bevelled surface at the top of each. 
Both limbs of sloths are remarkable for their length and 
slenderness, but the front pair are much longer than the 
hinder ones. The narrow and curved feet terminate in 
long hooked claws, which in the three-toed species are 
three in number in each foot, although in the fore-feet of 
