83 



Edentata which have just been compared with the Mylodon in this respect. 

 But these, like the Mylodon, combine with such a humerus all the essential 

 dental and osteological evidences of their close affinity to the phyllophagous 

 Edentata ; if, therefore, the locomotive extremities of the Megatherioids should 

 exhibit many marked deviations from those of the Sloths, whereby they approach 

 nearer to those of the insectivorous Edentata, it is to be regarded as the ine- 

 vitable consequence of their having to support bodies of far too great bulk and 

 weight to be suspended from the boughs of trees ; and as the teeth and jaws of 

 the gigantic sloth-like Edentals prove them to have fed on the foliage of trees, 

 they must have possessed limbs organized for a different and more violent mode 

 of obtaining such food, than are those of the existing Sloths, which have no other 

 resistance to overcome than the light weight of the body, which the long and 

 slender limbs are accordingly adapted to transport from bough to bough. 



In general form, and in the development of the proximal tuberosities, deltoidal 

 crest, and distal condyles, the humeri of the great Ant-eater, of the Orycterope, 

 and among Armadillos, that of the Chlamyphore, more nearly resemble the hu- 

 merus of the Mylodon ; but in aU these existing Edentals the internal humeral 

 condyle is perforated. 



With respect to this osteological character there is the same variety in the ex- 

 isting Sloths as in the extinct Megatherioids : the Ai, or three-toed species, has 

 an imperforate humerus, as in the Megatherium and Mylodon ; in the two-toed 

 Sloth the bone is perforated, as in the Megalonyx and Scelidotherium. The hu- 

 merus of the latter extinct Edental* resembles that of the Megalonyx and Mega- 

 therium in the less marked development of the deltoidal crest ; but in the size 

 and position of the proximal tuberosities, in the proportions of the shaft to the 

 extremities, and in the slight convexity of the ulnar division of the distal trochlea, 

 it more resembles the humerus of the Mylodon. 



In all the known extinct Megatherioids the humerus is destitute of a medul- 

 lary cavity, and is filled with a Hght spongy texture, as in the Sloths : this is a 

 more important mark of their natural affinity, than any which the superficial 

 muscular modifications of the bone might indicate. 



Ulnaf. — The proportions of the ulna of the Mylodon are such that it might 



* Fossil Mammalia of the Beagle, loc. cit. p. 90, pi. XXV. figs. 1 and 2. 

 t Plates XI. XII. XIII. fig. 2. 



l2 



