334 MEGATHERIOIDS. 



of the Megalonyx ; the entire tooth presents a double curvature, 

 its anterior and external sides being slightly convex, its posterior 

 and internal ones concave(l). This tooth consists, like the one 

 described by Cuvier, of a central body of vascular dentine, sur- 

 rounded by a thin layer of hard dentine, called enamel by Cuvier 

 and Dr. Harlan, and with an exterior coating of cement : the 

 central less dense axis is worn, as in the Ai's molars, into a hollow ; 

 but the hard boundary is uniformly obtuse, not raised into points. 



Mylodon. — In this genus of extinct phyllophagous Bruta the 

 teeth are eighteen in number, disposed, as in the dental formula 

 of the Sloths, 4-E5, or five on each side of the upper jaw, four on 

 each side of the lower jaw. They are, as usual, long, and of the 

 same thickness from the exposed to the implanted end : the latter 

 is excavated by a deep conical pulp-cavity, the exposed surface is 

 worn into a shallow depression, with a raised obtuse margin. In 

 the Mylodon robustus, — a species about the size of a Rhinoceros, 

 but in some dimensions equalling the Elephant, — the first tooth 

 of the upper jaw is separated by a marked interval from the rest, 

 and it is also more curved, the convexity being turned forwards : 

 its transverse section is subtriedal, with the angles rounded ; it 

 measures not more than ten lines across its longest diameter, and 

 is rather more than three inches in length, with a very small 

 portion protruding from the socket. The second molar has an 

 elliptical transverse section, with the Igng axis parallel with that 

 of the skull : it is of the same size as the first. The remaining 

 upper molars are triedral, indented along the side which looks 

 inwards ; the last, of rather larger size than the others, having 

 its longest diameter from behind forward, the other two with theirs 

 transversely: the four posterior molars have short interspaces. In the 

 lower jaw the first molar resembles that above in its size and curvature, 

 but it is nearer the second tooth, and has an oval transverse section ; 

 it plays upon the posterior part of the upper tooth, like the inferior 

 canine-shaped molar in the Unau. The second tooth is bent outwards, 

 but less so than the first ; its inner surface presents a longitudinal 



(ly Harlan's Medical and Physical Researches, Svo. 1835, pp, 323, 324, PI. xii. 

 figs. 7, 8, 9. 



