43 



molar region and of the corresponding part of the lower jaw, the lower border of 

 which 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 

 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 fossa: 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 the Mylodon, and the postorbital process of the malar bone is obsolete ; the orbit is 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 

 three 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 in the skull of the 

 Mylodon robustus 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 mandibular, 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 

 robustus, 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-f~. 



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 fossae 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 



* Memoir on the Mylodon, 4to, 1842, pi. 3. 



t Fossil Mammalia of the ' Voyage of the Beagle,' p. 73. pis. 20, 21, & 22. 



