152 FOSSIL MAMMALIA. 
or Rhinoceros ; and the ui^per convex surfaces 
of 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, f,) 
resembles that of no other family except the 
Sloths, and exhibits in the Acromion (g,) con- 
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, shows that the 
fore-leg discharged some other office, than that 
of an organ of locomotion. This clavicle Mould 
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 habits 
of the Megatherium ; 1°. a free rotatory power 
of the arm was auxiliaiv to its office, as an 
