PROFESSOR OWEN ON THE MEGATHERIUM. 
585 
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, from 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 
molar region and of the corresponding part of the lower jaw, the lower border of 
whieh is consequently nearly straight in the Mylodon, as it is in the existing Sloths. 
So great a proportional extent of the descending process of the malar bone is con- 
sequently not required, and this process is more oblique in direction, and is relatively 
broader and thinner than in the Megatherium. In these differences also the Mylo- 
don shows its closer resemblance to the Sloths. The basioccipital is relatively 
broader, and the occipital condyles are wider apart in the Mylodon ; the occipital 
plane is more inclined and the zygomatic process proportionally weaker than in the 
