36 



It remains, then, to inquire whether, among the extinct forms of the Mamma- 

 han class to which was assigned the office of restraining the too luxuriant vege- 

 tation of a former world, there be any that, from their cranial or dental charac- 

 ters, may be concluded to have resembled the Mylodon in the mode of perform- 

 ing that task ? 



The skull of the Megatherium, while it presents all the essential resemblances 

 to that of the Mylodon which have been pointed out in the skull of the Sloth, 

 as the long cranium, the short muzzle, the terminal position of the condyles, 

 and of the occipital and nasal apertures, and the large and compHcated malar 

 bone, approximates still more closely in the junction of the malar with the zy- 

 gomatic process of the temporal, and in the relative depression and flatness of 

 the elongated cranium. 



In thus receding from the Sloths the Megatherium does not approach any 

 other existing genus, but only another member of its own peculiar extinct 

 family ; and in receding from the Mylodon in the cranial characters next to be 

 noticed, the interval is not diminished which separates the Megatherium froni 

 existing large vegetable feeders, but its own peculiar modifications alone are 

 manifested. 



The most marked differences in the skulls of the Megatherium and Mylodon 

 depend on the greater length of the teeth, and consequent depth of their sockets, 

 in the larger species ; which require a greater vertical extent of the maxillary 

 bone in the molar region, and of the corresponding part of the lower jaw, the 

 lower border of which describes a deep convex curve at the middle third of the 

 horizontal ramus. This modification necessitates a greater proportional length 

 of the descending process of the malar bone, which is at the same time more 

 vertical in position and relatively narrower than in the Mylodon. In these dif- 

 ferences the Mylodon shows its closer resemblance to the Sloths. 



The basi-occipital is relatively narrower, and the condyles nearer together, 

 in the Megatherium than in the Mylodon ; the occipital plane is less inclined ; 

 the zygomatic process is proportionally stronger. The greater depth of the 

 lower jaw and the heavier grinding machinery call for a more extensive origin 

 of the temporal muscles ; and accordingly the superior boundaries of their fossae 

 meet and form a median crest upon the upper surface of the cranium of the 

 Megatherium. This structure, which is represented by MM. Pander and 



