HERBIVOROUS TYPES. 



127 



be seen at a glance that the dentition is quite different 

 from that of either of the carnivorous types figured above. 

 The lower teeth comprise a single large incisor (a), behind 

 which were either three or four tall premolar teeth with 

 cutting edges, and marked on the sides with a number of 

 oblique grooves, from which the name Plagiaulax was 

 taken. When unworn, these grooves extended along the 

 whole outer surface of the teeth, but when the teeth had 

 been long in use (as in our figure) the groovings became 

 worn away from the sides. Behind these four premolars 

 are two smaller molar teeth, with the summits of their 

 crowns marked by a single longitudinal groove bounded 

 by prominent ridges. Now it was argued at first that 

 this very peculiar type of dentition indicated carnivorous 

 habits in the owners thereof; but it was subsequently 

 pointed out that the existing rat-kangaroos of Australia 

 (of which the front of the skull is shown in Fig. 42) 



presented a some- 

 what similar type 

 of tooth- structure. 

 Thus, the last pre- 

 molar tooth (p.m.) 

 of the rat-kanga- 

 roo has a cutting- 

 crown marked 

 with a number of 

 parallel grooves; 

 while each half 



IG. 42. Jaws and Teeth of Eat-Kangaroo. ? f ^ l wer . J ' aW 



terminates in a 



single large incisor not unlike that of the Jurassic Plagiau- 

 lax. Hence it was argued and, in our opinion, argued 

 rightly that as the living form is herbivorous, the same 

 must have been the case with the extinct one. When, 

 however, it was also urged that the rat-kangaroo and 

 Plagiaulax were closely allied animals, important differences 

 between the two were overlooked. For instance, as will 

 be apparent from the figures, while in the former there 

 was but one grooved tooth, in which the grooves are 

 vertical, in the latter there were usually three or four 



