40 



greatest at the part corresponding with the posterior part of the cerebral 

 hemispheres, is six inches : from these indications it may be concluded 

 that the brain of the Megatherium was flatter or more depressed, and 

 smaller by nearly one-half than that of the Elephant; but that the cere- 

 bellum was relatively larger and situated more posteriorly with relation 

 to the cerebral hemispheres. 



From Punta Alta, Bahia Blanca, Patagonia. 



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



The following specimens of the teeth of the Megatherium will be described 

 in detail, as no complete account of the dentition of this remarkable extinct 

 animal has hitherto been published ; and such a description may be most 

 essential to the determination of the species — if there be more than one — of 

 the genus. 



222. The first right molar tooth of the upper jaw. This is the second in 



point of size, the last being the least. It is eight inches and a half in 

 length ; the pulp-cavity extends five inches from the base : it presents 

 two slight curvatures, one having the convexity turned forward, and the 

 other inward. The transverse section gives an irregular semicircle, 

 with the convexity turned forward, and the flat side next the second 

 tooth : the angles at which this joins the curve are rounded ; the outer 

 angle is somewhat produced, and the outer side of the curve is flattened. 

 The grinding surface presents two transverse ridges, one close to the 

 anterior surface, the back part of which is nearly vertical ; the other near 

 the posterior margin with a nearly vertical posterior slope, and a longer 

 and more oblique anterior one. The central axis of the tooth, formed 

 by the vascular dentine, is irregularly tetragonal : the cement is thick on 

 the anterior and posterior surfaces, thin on the sides of the tooth. 

 From the Rio Salado, Buenos Avres. 



Presented by Sir TV. Parish, K. H. 



223. The second right molar of the upper jaw. This is the largest of the 



upper teeth, and is upwards of nine inches in length, of a tetragonal form, 

 with two slight curvatures, as in the preceding tooth. The posterior 



