42 



the proportions of those in the Mylodon, but is not separated from the rest by 

 so wide a diastema. In neither of the existing Sloths has the last molar of the 

 lower jaw so complicated a form or so large a relative size as in the Mylodon, 

 but the three-toed species makes the nearest approach to this character in the 

 outer and inner longitudinal grooves of the last molar. 



The superior number, and still more the distinct microscopic structure of the 

 teeth of the Orycterope and Armadillos, remove them from the pale of com- 

 parison with the Mylodon and its extinct congeners. 



The comparison of the dentition of the species under consideration with that 

 of the Mylodontes Darwinii and Harlani is necessarily limited to the teeth oi 

 the lower jaw. 



With respect to the Mylodon Harlani, the difference is chiefly manifested in the 

 last molar tooth. In the Mylodon robustus the outer side of this tooth is impressed 

 with a single wide and shallow longitudinal depression ; but in the Mylodon 

 Harlani it presents two similar depressions, the anterior one being the deepest. 

 The deep and broad posterior longitudinal depression on the inner side of this 

 tooth is round in Mylodon robustus, but angular in Mylodon Harlani. 



In Mylodon Darwinii the posterior molar of the lower jaw is less complicated 

 and relatively smaller than in either of the other species ; being impressed by 

 only a single longitudinal channel on both the outer and inner sides : the inner 

 channel is as oblique as the outer one, but it penetrates the tooth in the opposite 

 direction. 



The penultimate molar in the Mylodon Darwinii has both the outer and inner 

 side longitudinally indented, but not the anterior side, as in the Mylodon robustus. 

 The antero-posterior diameter of the crown exceeds the transverse diameter, 

 whilst the corresponding tooth of the Mylodon robustus has these proportions 

 reversed. The second or antepenultimate molar is less deeply indented along 

 the inner side. The first molar has a simple elliptical transverse section, as in 

 the other species of Mylodon. 



The dentition of the genus Megalonyx is at present known only (with the ex- 

 ception of the lower jaw, brought home by Mr. Darwin,) by the detached teeth 

 figured by Cuvier in Megalonyx Jeffersonii, and by Dr. Harlan in his Megalonyx 

 laqueatus. Either of these detached teeth sufficiently indicate by their com- 

 pressed figure, giving in transverse section a long, irregular ellipse, with a pro- 



