152 
FOSSIL MAMMALIA. 
or Rhinoceros ; and the upper convex surfaces ot 
some of them exhibit a rugous and flattened 
condition of that part, on which the weight of 
a bony cuirass would most immediately have 
rested. 
Anterior Extremity. 
The scapula or shoulder blade, (PI. 5, Fig. 1 , Q 
resembles that of no other family except the 
Sloths, and exhibits in the Acromion (g,) coii' 
trivances for strength, peculiar to itself and 
them, in its mode of articulation with the collar 
bone (h) ; it exhibits also unusual provisions 
for the support of the most powerful muscles for 
the movement of the arm. 
The clavicle or collar bone (h) is strong, and 
curved nearly as in the human subject; the 
presence of this bone in the Megatherium, whilst 
it is wanting in the Elephant, Rhinoceros, and 
all the large ruminating animals, shews that the 
fore leg discharged some other office, than that 
of an organ of locomotion. This clavicle would 
give a steady and fixt position to the socket, or 
glenoid cavity of the scapula, admitting of rota- 
tory motion in the fore leg, analogous to that of 
the human arm. There is in these circumstances 
a triple accommodation to the form and habit® 
of the Megatherium ; 1". a free rotatory power 
of the arm was auxiliary to its office, as an 
