MEGATHERIOIDS. 335 



indentation ; its transverse section is triangular, and the posterior 

 and inner angle is produced. The third molar presents a sub- 

 quadrate transverse section, placed obliquely in the jaw : it is of 

 the same size as the second. The fourth and last molar is the 

 largest, and has the most complex form of any in the series : it 

 is slightly bent lengthwise with the convexity turned inwards, 

 and by the longitudinal channels which traverse its outer and 

 inner surfaces, the transverse section or grinding surface is made 

 bilobed : the inner channel is oblique and much deeper than the 

 outer one ; the anterior surface of the anterior lobe of the tooth 

 is also impressed by a shallow channel. The grinding surface 

 of all these teeth is nearly flat ; wdth the slight central depression 

 bounded by an obtuse margin: the small anterior teeth, which, 

 by their more advanced position and curvature, are analogous to 

 the large laniariform molars of the Unau, are rather obliquely worn. 

 The teeth of the Mylodon Harlani, very closely resemble those of 

 the Myl. robustus in figure : the last molar of the lower jaw presents 

 the most characteristic difference : it has two shallow longitudinal 

 channels along its outer side, the anterior one being the deepest : 

 and the deep and broad longitudinal channel on the inner side 

 of the tooth is angular, and not rounded as in the Myl. Robustus. 

 In the Mylodon Darwinii, a third species of this extinct genus, 

 the posterior molar of the lower jaw is relatively smaller than in 

 either of the other species, and is impressed with a single longi- 

 tudinal channel, on both the outer and inner sides ; both penetrate 

 the tooth obliquely, but in opposite directions ; and both are equally 

 deep : the anterior lobe is not impressed by any channel. The penul- 

 timate molar in the same species has the inner side longitudinally 

 indented ; but not the anterior side, as in the Mylodon Harlani : 

 the second molar is less deeply indented along the inner side(l). 



I have investigated microscopically the structure of the teeth in 

 the last two species of Mylodon, and have given illustrations of that of 

 the Myl. robustus in the description of the skeleton of that species 

 published by the Royal College of Surgeons (2). Each tooth consists, 

 as in the Sloth, of a central axis of vascular dentine, enclosed by a 



(1) PL 80, fig. 5. (2) Memoir on the Mylodon, &c., 4to. 1842, PI. xxiv. figs. 2 and 3. 



