586 
PROFESSOR OWEN ON THE MEGATHERIUM. 
Megatherium. The minor depth of the lower jaw and the lighter grinding instru- 
ments call for a less extensive origin of the temporal muscles, and accordingly the 
superior boundaries of their fossse are separated in the adult Mylodon by a wide and 
smooth parietal tract*, as in the Sloths. The postfrontal process is rudimentary in 
the Mylodon, and the postorbital process of the malar bone is obsolete ; the orbit is 
consequently without any bony boundary behind ; the malar, accordingly, has but 
th ree processes, and is thus less complicated than in the Megatherium. The inter- 
orbital part of the skull is relatively narrower, the maxillary part relatively broader, 
than in the Megathere. No trace of premaxillaries was present iti the skull of the 
Mylodon i^ohustus described by me, and the broad truncated symphysis of the lower 
jaw indicates that they must have been very small if they existed : the peculiar length 
of the premaxillaries in the Megatherium, and the corresponding prolongation of the 
long and narrow symphysis mandibulm, offer the most conspicuous differences in the 
conformation of the skull, and proportionally remove that genus from the existing 
Sloths. The bony palate is absolutely broader in the much smaller skull of the 
Mylodon than in the Megatherium, and it gradually contracts from the first to the 
fifth molar : it is, e. g. 5 inches in breadth between the first molars in the Mylodon 
rohustus, and only 2 inches in breadth between the same teeth in the Megatherium. 
The opportunity of a comparison of the skull of the Megalonyx with that of the 
Megatherium is yet a desideratum : it would probably demonstrate some inter- 
mediate modifications between the latter and the Mylodon. 
The extinct megatherioid animal of which, after the Mylodon and the Megathe- 
rium, the most complete cranium has hitherto been obtained, is the Scelidotherium-J'. 
In many respects the skull of this animal resembles that of the Megatherium more 
than does that of the Mylodon. The plane of the occiput is rather less inclined 
from below forward than in the Megatherium, but more resembles that part than in 
the Mylodon : the upper boundaries of the temporal fossse more nearly approximate 
than in the Mylodon : the bony palate is narrower, and its sides more parallel than in 
the Mylodon ; but instead of being concave transversely, as in the Megatherium, it 
is convex: the alveoli are nearer together than in the Mylodon, and the first is not 
separated by a wider diastema than the rest. The symphysis of the lower jaw is much 
prolonged, but is less deeply channelled above than in the Megatherium, and is not 
so distinctly defined by the abrupt increase of depth of the ramus behind it which 
characterizes the Megatherium ; the molar part of the mandible makes, however, a 
greater convexity below than in the Mylodon. 
With these marks of approximation to the Megatherium there are, however, the 
same differences as in the Mylodon in regard to the widely open orbit, the more simple, 
trifurcate, malar bone, the minor depith of the alveolar portions of the jaws, and the 
straighter outline of the lower border of the mandible. In both the Mylodon and 
* Memoir on the Mylodon, 4to, 1842, pi. 3. 
t Fossil Mammalia of the ‘ Voyage of the Beagle,’ p. 73. pis. 20, 21, & 22. 
