338 . MEGATHERIODS. 



Mylodon rohustns ; but the difference is less, as compared with 

 the lower molars in the Mylodon Darwinii. The two series of 

 upper molar teeth are separated by a narrower palate in the 

 Scelidothere than in the Mylodon. The first molar in the upper 

 jaw, (PL 80, fig. 1), is trihedral, and much less curved, and more 

 compressed than in the Mylodon. The second molar is also 

 three-sided, and its transverse section is triangular instead of 

 elliptical ; and two of its sides are slightly indented : it resembles 

 the antepenultimate molar in the Myl. rohustus. The third and 

 fourth molars of the Scelidothere are more compressed, and the 

 long axis of their transverse section is oblique, whilst in the Mylodon 

 it is transverse to the line of the jaw. The fifth molar, besides 

 being relatively smaller, has a trihedral figure with the broadest 

 side turned outwards, and is slightly excavated lengthwise. 



In the lower jaw, the differences in the form of the teeth are 

 of the same degree : a comparison of figure 2 with figure 5, in 

 Plate 80, will demonstrate their nature in the Seel, leptocephalum, 

 and Mylodon Darwinii. The lower molars of the Myl. rohustus and 

 Myl. Harlani differ still more, especially in the last species, from those 

 of Scelidotherium. 



The composition of the teeth closely resembles that in the genus 

 Mylodon, but the central axis of vascular dentine is relatively smaller : 

 it is traversed, however, as in the Mylodon by medullary canals, 

 which, at the periphery of the axis anastomose by loops, from 

 the convexity of which the calcigerous tubes are given off, which 

 penetrate and constitute so large a proportion of the hard dentine. 

 This substance is about one line and a half in thickness, and is 

 invested by a coat of cement, not exceeding one third of a line 

 in thickness. The teeth of the Scelidothere are thus calculated to 

 offer more resistance than those of the Mylodon, having a larger 

 proportion of the hard unvascular dentine, by which they approach 

 nearer to the structure of the teeth in the Armadillo tribe. 



Megatherium. — The teeth of this most gigantic of the extinct 

 quadrupeds of the Sloth-tribe are as small in proportion to the 

 size of the animal as in the Mylodon ; they are five in number 

 on each side of the upper jaw, and, probably, four on each side of 



