39 



The relations of the Megathere and Scelidothere to the Mylodon in the form 

 of the lower jaw will be better appreciated when the modifications of this bone 

 in the different species of Mylodon have been pointed out. In the Mylodon 

 Darwinii the rami of the lower jaw are relatively longer than in the Mylodon 

 robustus, especially anterior to the molar series, where they become more con- 

 tracted vertically, and converge to a narrower and longer symphysis. The 

 posterior angular process is relatively shorter and more bent upwards. The 

 molar teeth project further from their sockets in the specimen compared with 

 that of the Mylodon robustus. The anterior outlet of the dental canal is single, 

 and it is more in advance of the first alveolus. The symphysis is not only 

 longer, but is incUned forwards at a more open angle with the horizontal ra- 

 mus : the two tuberosities on the outside of the symphysis for the attachment 

 of the retractors of the lower lip are much more strongly developed in the 

 Alijlodon Darwinii, than in the Mylodon robustus. 



The lower jaw of the Megatherium is relatively narrower than in the Mylo- 

 don, but deeper, except at its anterior or symphysial prolongation, which is 

 much more, and more suddenly contracted in all its dimensions. The angle of 

 the jaw is less produced backwards, so that the Mylodon Darwinii is somewhat 

 intermediate in this respect between the Mylodon robustus and the Megatherium : 

 it wants, however, Uke its congener, the deep convexity of the submolar part of 

 the jaw, which has already been alluded to as characteristic of the Megatherium, 

 in which it gives greater depth to the sockets of the teeth. The symphysis 

 slopes forwards almost horizontally in the Megatherium : its upper margin is 

 expanded and convex on each side. The muscles of the lower Up which took 

 their origin from the symphysial tuberosities in the Mylodon, probably arose 

 from the submolar convexities of the lower jaw in the Megatherium. 



The Scelidotherium resembles the Mylodon in the general form of the lower 

 jaw : the angular process is most like that in Mylodon robustus, but is more ob- 

 tuse. The horizontal ramus becomes narrower vertically as it approaches the 

 symphysis, and thus more resembles that of the Mylodon Darwinii. In all the 

 essential characters of the lower jaw, as in the number, structure, and kind of 

 teeth which it contained, the extinct Megatherioid quadrupeds more closely re- 

 semble each other and the existing Sloths, than any other known existing or 

 extinct species. 



