TOXODON. 583 



molar, the last but one on the right side of the upper jaw. This 

 tooth 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 Wombat. 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. The form of the grinding surface of 

 this molar is shewn in PI. 86, fig. 4. 



All the molar teeth are long and curved, and without fangs, as in 

 the Wombat and most of the herbivorous species of the Rodent order : 

 in those, however, with curved grinders, as the Aperea, or Guinea-pig, 

 the concavity of the upper grinders is directed outward, the fangs of the 

 teeth of the opposite sides diverging as they ascend in the sockets ; but 

 in the Toxodon the convexity of the upper grinders is outward, as in 

 the Horse, but with so much greater curvature that the fangs converge 

 and almost meet at the middle line of the palate, forming a series of 

 archeSj capable of resisting great pressure. It is this structure which 

 suggested to me the generic term proposed for this most remarkable 

 extinct Mammal. (1) 



Of the upper incisors, the two small ones are situated in the 

 middle of the front of the intermaxillaries, and the two large ones in 

 close contiguity with the small incisors, which they greatly exceed in 

 size. The sockets of the two large incisors extend backwards, in an 

 arched form, preserving a uniform diameter, as far as the commence- 

 ment of the alveoli of the molar teeth ; the curve which they describe 

 is the* segment of a circle ; the position, form, and extent of the 

 sockets are such as are only found in those of the corresponding teeth 

 of the Rodentia among existing Mammalia. The matrix, or forma- 

 tive pulp of the large incisors was lodged, as in the Rodentia, in close 

 contiguity with the sockets of the anterior molars ; and we are enabled 

 to infer, from the form of the socket, notwithstanding the absence of 



tl) To^oy, areas ; dhovi, dens. ' Zoology of the Beagle, Fossil Mammalia,' 4to. 1839, p. 16. 



