﻿MULTITUBERCULATA. 285 



as shown in the accompanying cut, are of relatively large size, 

 and are characterised, when unworn, by having a highly com- 

 pressed, convex crown, with its summit narrowed to a sharp 

 cutting edge, and its sides usually marked by oblique ridges 

 and grooves. The single pair of lower incisor teeth are curved 

 and pointed. 



It was from the resemblance of their remarkable premolar 

 teeth to the last premolar tooth of the Rat-Kangaroos, coupled 

 with the presence in both groups of a single pair of large 

 lower incisors, that led to the Multituberculata being included 

 among the Diprotodont Marsupials. An important point of 



One half of an imperfect Lower Jaw of a North American Representa- 

 tive of the Plagiaulacidee. Much enlarged. (After Marsh.) 



difference between the two groups is, however, to be found in 

 the circumstance that whereas in the Rat-Kangaroo the summit 

 of the crown of the last premolar is slightly concave, in all the 

 PlagiaulacidcB it is markedly convex. 



It is noteworthy that the earlier members of the family ha\e 

 a large number of premolars, and but few grooves on the last 

 tooth of that series, whereas in the later forms the number of 

 premolars diminishes, while the grooves on the last premolar 

 become more numerous. 



The nature of the food of Plagiaulax^ all the species of 



