88 ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



for its own mere support requiring therefore no such mechanism, was used in aid 

 of the fossorial actions of the extremities. 



As the cervical vertebrae of the Megatherium have their processes compara- 

 tively weaker than in the Scelidotherium, and the anterior dorsal spines are rela- 

 tively shorter, it may be concluded, that whatever were the extent or nature of the 

 fossorial labours of the enormous claws with which it was provided, the head did not 

 co-operate with the digging implements in their especial task in the same degree 

 as in the Scelidothere and Orycterope. At the same time there is no modification 

 of the cervical region of the spine of the Megathere corresponding with those which 

 we have seen to be subservient to the arboreal habits of the sloth, a remark which 

 will not be deemed superfluous by those who have perused the acute observations 

 and arguments adduced by M. Lund in favour of the scansorial character of the 

 extremities of the Megatherium and Megalonyx. 



The fragments of the dorsal vertebra? and ribs of the Scelidotherium, which are 

 figured in Plate XX, offer no modifications which need detain our attention ; they 

 closely conform, excepting in the greater relative height of the anterior dorsal spines, 

 already noticed, with the Megatherioid type. The sacrum manifests in its vast 

 expanse, the great development of the posterior transverse processes to join the 

 ischium, the capacious medullary cavity, and wide nervous foramina, a like con- 

 formity with the Megatherium, and a corresponding harmony with the dispropor- 

 tionate bulk of the hind legs. 



Bones of the Extremities. 



The Scapula in its double spine, the osseous arch formed by the confluence 

 of the acromion with the coracoid process, and the substitution of a distinct foramen 

 for the suprascapular notch, agrees with that of the Megatherium : but the span 

 of the acromial arch is relatively wider, and the surface for the articulation of the 

 clavicle is better marked. This articular surface, which is distinctly shewn upon 

 the acromion of both the scapulae in PI. XX. is the more interesting, as being 

 the only evidence of the clavicle of the Scelidothere which we at present possess; 

 but it is enough to prove that this quadruped enjoyed all the advantages in 

 the actions of the fore-extremity, which arise out of the additional fixation of the 

 shoulder-joint afforded by the clavicle— a bone which the extinct Megatherioids 

 are the largest of the mainmiferous class to possess in a completely developed 

 state. The form, position, and aspect of the glenoid cavity for the humerus 

 closely correspond with the condition of the same part in the Megatherium. The 

 limits of the acromial and coronoid portions of the arch were still defineable in the 



