42 



with that of the Megatherium, are the greater relative depth and breadth, and the 

 more convex outline of the coronal aspect of the skull ; but this difference would be, 

 doubtless, much less marked in the immature than in the adult Megatherium. The 

 zygomatic process, in the Sloths, is relatively shorter, and does not attain the malar 

 bone ; this, therefore, has not the middle process for supporting the zygoma, and is two- 

 pronged, instead of being, as in (he Megatherium, four-pronged. The chief characters 

 by which the Megatherium deviates, in its cranial structure, from the bradypodal and 

 approaches to the myrmecophagal type, are the elongation of the slender edentulous 

 fore-part of the upper jaw, and of the corresponding grooved slender edentulous part 

 of the lower one: but the prolongation of the upper jaw is due to relatively longer 

 premaxillaries than are developed in any of the true Edentata. The zygomatic 

 arches, moreover, are more defective in the Anteaters and Pangolins than in the 

 Sloths ; the malar part especially being minute or obsolete. Only in the Orycterope 

 and Armadillos, amongst the existing Bruta, is the zygomatic arch complete, but it is 

 simple, without ascending or descending processes. The great Glyptodon, indeed, 

 exemplifies that, tendency to community of characters so often presented by extinct 

 species, in an inferior prolongation of the malar bone analogous to that of the Sloths 

 and Megatherium. With other existing mammals than those of the Edentate order, 

 it would be lost time to pursue the present comparison with a view to the elucidation 

 of the affinities of the Megathere. It needs only to place the skull of this animal by 

 the side of those of the Elephant, Rhinoceros, Sivatherium, Ox, Elk, Horse, Dugong, 

 or other vegetable-feeding mammal of corresponding or approximate size, to be struck 

 with the peculiarities of the fossil, and to be convinced that the habits and mode of 

 feeding of the Megatherium had been such as are no longer manifested by the larger 

 Herbivora of the present day. 



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

 class to which was assigned the office of restraining the too luxuriant vegetation of a 

 former world, there be any that, fiom their cranial or dental characters, may be con- 

 cluded to have resembled the Megatherium in the mode of performing that task. 



The skull of the Mylodon, while it presents all the essential resemblances to that of 

 the Megatherium which have been pointed out in the skull of the Sloth, as, e.g. the 

 long cranium, the terminal position of the occipital condyles, and of the occipital 

 and nasal apertures, and the large and complicated malar bones, approximates still 

 more closely to the Megatherium in the junction of the malar with the zygomatic 

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

 cranium. But in thus receding from the existing Sloths, the Mylodon does not 

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

 extinct family. 



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

 pend on the minor length of the teeth and consequent depth of their sockets in the 

 smaller species, which require a less vertical extent of the maxillary bone in the 



