41 



and broadest side is nearly flat, the anterior side somewhat convex, the 

 outer and narrowest side is concave, the inner side is sinuous, having 

 a median longitudinal eminence between two longitudinal concavities. 

 The central axis of vascular dentine is more compressed from before 

 backwards than in the preceding tooth, and its posterior surface is con- 

 cave : the two transverse ridges of the grinding surface of the tooth are 

 nearly equal, but the sloping side formed by the dentine is larger than 

 that formed by the cement. 



From the Rio Salado, Buenos Ayres. 



Presented by Sir W. Parish, K. H. 



224. A portion of the corresponding molar tooth of a larger Megatherium. 



From the province of Buenos Ayres. Purchased. 



"22b. The second left molar of the upper jaw. 



From the Rio Salado. Presented by Sir W. Parish, K.H. 



'226. The third left molar of the upper jaw. It nearly resembles in form the 

 second, but the anterior and outer angle is less rounded off, and the ex- 

 ternal longitudinal depression rather more marked. 



From the Rio Salado. Presented by Sir W. Parish, K.H. 



22~- Portions of the second and third left molars of the upper jaw, with por- 

 tions of the sockets. 



From Punta Alta, Bahia Blanca, Patagonia. 



Presented by Charles Darwin, Esq., F.R.S. 



228. This tooth so closely agrees with the first molar of the upper jaw, that 

 if it be not the corresponding tooth of another species of Megatherium, 

 it must be referred to the corresponding place in the lower jaw. The 

 anterior surface is less convex than in the upper molar, and the entire 

 tooth presents a tetragonal, rather than a semi-cylindrical form ; the an- 

 terior facet being, however, half the breadth of the posterior one, by 

 which it differs from all the tetragonal teeth of the upper jaw. Both the 

 inner and outer sides which converge to the anterior surface are slightly 

 concave, whilst the inner side, on the first upper molar, is convex. The 



G 



