238 MOSTLY MAMMALS 
of the ordinary skin of the body, which extends outwards 
between the limbs as far as the wrists and ankles. In 
addition to the two lateral membranes, there is a narrow 
and inconspicuous one passing from each cheek along the 
front of the shoulder to the front of the wrist; and another, 
at least in the larger forms, connecting the two hind-legs 
and involving the base of the tail. 
In general characters the parachute of the scale-tailed 
flying-squirrels of Africa conforms to the above type; and 
a superficial observer might say that the two were in all 
respects similar. A closer examination will, however, reveal 
the fact that the parachute in this group is supported by 
a process of cartilage projecting like a yard-arm from the 
elbow and extending to the edge of the membrane. As 
this is present in all the scale-tails (as we may call them 
for short, especially as they have no right at all to the 
title of squirrels) and absent in all the true flying-squirrels, 
it evidently indicates an important difference between the 
two groups. 
A further important distinction between them is afforded 
by the presence on the under-surface of the basal portion of 
the tail of a series of overlapping horny scales, from which 
the African group takes both its popular title of scale-tail 
and its scientific name of Axomalurus. Evidently these 
scales are intended to aid in supporting the animals as 
they climb the boughs or stems of trees, and they are 
thus strictly analogous to the stiff tail-feathers of wood- 
peckers. 
Yet another difference between the two groups is to be 
found in the structure of the crowns of their cheek-teeth. 
In ordinary squirrels the grinding surfaces of these teeth 
are surmounted by simple tubercles, which in some cases 
may be elongated into ridges. And a similar type of 
