130 



561. The sixth molar tooth, left side, upper jaw of the Toxodon platensis. 



It is curved, with the convexity turned outwards when lodged in the 

 socket, contrary to the position of the superior curved molars in the 

 Guinea-pig and Womhat. The outer surface of the tooth is traversed by 

 two slight convex longitudinal risings : the inner side presents, anteriorly, 

 a slightly concave surface, and posteriorly two prominent longitudinal 

 convex ridges separated by a deep channel, which is flat at the bottom : 

 a fold of enamel is continued from the anterior angle of this channel 

 obliquely forwards half-way across the body of the tooth. The outer 

 coat of enamel is interrupted at the anterior and posterior margins 

 of the grinder. 



From the tertiary deposits in the banks of the Rio Tercero near the 

 Parana, South America. 



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



562. A portion of the right ramus of the lower jaw of the Toxodon platensis. 



563. A smaller portion of the left ramus of the same lower jaw of the Toxodon 

 platensis. 



These were discovered at Bahia Blanca, in latitude 39°, on the east 

 coast of South America. 



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



The molar teeth in this mutilated lower jaw, like those in the upper 

 jaw of the Toxodon, had persistent pulps, as is proved by the conical 

 cavity at their base : they consequently required a deep socket and a cor- 

 responding extent of jaw to form the sockets and protect the pulps. In 

 order to economize space and to increase the power of resistance in the 

 tooth, and perhaps also to diminish the effects of direct pressure on the 

 highly vascular and sensible matrix, the molars and their sockets are 

 curved, but in a less degree than those of the upper jaw of the Toxodon, 

 They correspond, however, with the superior molars of the Toxodon in 

 the antero-posterior diameter, in being small and simple at the anterior 

 part of the jaw, and by increasing in magnitude and complexity as they 

 are situated more posteriorly. They are, however, narrower from side to 



