122 FOSSIL MAMMALIA. 



organ of locomotion. This clavicle would give a steady 

 and fixed position to the socket, or glenoid cavity of the sca- 

 pula, admitting of rotatory 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 Mega- 

 therium ; 1°. a free rotatory power of the arm was auxiliary 

 to its office, as an instrument to be employed continually in 

 digging food out of the ground; 2°. this act of perpetual dig- 

 ging in search of stationary objects like roots, required but 

 little locomotive power; 3°. the comparatively small sup- 

 port afforded to the weight of the body by the fore-leg, was 

 compensated by the extraordinary and colossal strength of 

 the haunches and hind legs. In the Elephant, the great 

 weight of the head and tusks require shortness of neck, and 

 unusual enlargement and strength in the fore-legs ; hence, 

 the anterior parts of this animal are much stronger and 

 larger than its hinder parts. In the case of the Megatherium, 

 the relative proportions are reversed; the head is compa- 

 ratively small, the neck is long, and the anterior part of the 

 body but slightly loaded in comparison with its abdominal 

 and posterior regions. In the shoulder blade and collar 

 bone there is great provision to give strength and motion to 

 the fore-legs ; but this motion is not progressive, nor is the 

 strength calculated merely to support the weight of the 

 body. The humerus, (k) articulates with the scapula by a 

 round head, admitting of free motion in various directions, 

 and is small at its upper and middle part, but at its lower 

 end attains extraordinary breadth, in consequence of an 

 enormous expansion of the crests, which rise from the con- 

 dyles, to give origin to muscles for the movement of the fore- 

 foot and toes.* The ulna (1) is extremely broad and pow- 

 erful at its upper extremity, affording large space for the 



* There is a similar expansion of the lower part of the Humerus in the 

 Ant-eater, which employs its fore-feet in digging up the solid hills of the 

 Termite Ants. 



