37 



D'Alton*, is also very clearly indicated by the anterior portion of the skull of 

 the Megatherium in the Museum of the Royal College of Surgeons, in which the 

 anterior portions of the temporal ridges, where they diverge to the upper parts 

 of the orbits, are shown. The orbit in the Megatherium is better defined than 

 in the Mylodon, by the presence of a postorbital process ascending from the 

 upper margin of the malar bone, which bone is thus divided into four processes, 

 and is by so much the more compUcated than in the Mylodon. The inter- 

 orbital part of the skull is relatively broader, the maxillary part relatively 

 narrower, than in the Mylodon. There are three suborbital foramina in the 

 Megatherium, instead of one, as in the Mylodonf. The intermaxillary bones, 

 which are preserved in the above-mentioned skull of the Megatherium, are un- 

 questionably of a different form and of larger relative size than they could have 

 been in the Mylodon. This is proved by a comparison of the anterior margin 

 of the maxillary bqnes, to which they are articulated, with the correspond- 

 ing part of the skull in the Mylodon. In the latter these margins are nar- 

 row, rounded, smooth, and offer no trace of a sutural surface; in the Mega- 

 therium they present a broad rough depression, into which the base of the in- 

 termaxillary bone is wedged. The intermaxillary is an elongated, four-sided, 

 subcompressed bone, expanded at both extremities, united with its fellow, 

 bounding the anterior palatine aperture, which is absolutely narrower than in 

 the Mylodon, and prolonging the upper jaw ; to which the intermaxillaries thus 

 give a conical form, with a slightly expanded apex. This, perhaps, is the most 

 essential difference between the Megatherium and Mylodon, and one which 

 again removes the Megatherium farther from the true Sloths. Another well- 

 marked difference in the Megatherium is the absolutely narrower palate : the 

 intermolar part, for example, is widest between the anterior molars in both fos- 

 sils, but while its breadth at this part is five inches in the Mylodon, it is only 

 two inches in the Megatherium. The edentulous anterior part of the lower jaw 

 corresponds in length and narrowness with the upper jaw in the Megatherium, 

 and consequently offers another striking deviation from the cranial characters of 

 the Mylodon. 



• Das Riesen-Faulthier, pi. 3. fig. 3. 



t These holes indicate the nerves of the upper lip of the Megathere to have been of the same pro- 

 portional size as in the Tapir. 



