GEOLOGICAL DISTRIBUTION 



lower jaw (Fig. 25) and of one larger incisor, and in some instances 

 of one or two smaller ones in each premaxilla (Fig. 24). These 

 incisors are separated by an interval or diastema from the first of 

 the premolars. The true molars, and in some instances the pre- 



molars (Fig. 24), are 

 characterised by having 

 longitudinal rows of 

 tubercles separated by 

 one or more grooves ; 

 there being either two 

 or three of these rows 

 in the upper molars of 

 those forms in which 

 these teeth are known, 

 while there are, at least 

 usually, only two in 

 those of the lower jaw. In other cases the premolars are of a 

 secant type, with a highly convex cutting-edge, and usually either 

 serrated or obliquely grooved (Figs. 25, 26). From a certain 

 resemblance between these secant premolars and those of some of 

 the smaller Macropodidce it was at one time considered that we had 

 in these mammals representatives of Diprotodont Marsupials. The 

 great difference in the structure of the molar teeth of these forms, 



Fio. 25. The right ramus of the mandible of Plagiaulax 

 beklesl ; from the Purbeck of Swanage. Twice natural size. 

 i, Incisor ; m, molar ; 6, coronoid process ; c, condyle. (After 

 Owen.) 



Fio. 26. The imperfect right ramus of the 

 mandible of Plagiaulax minor ; from Swanage. 

 Four times natural size, p, Premolars ; m, 

 molars. (After Lyall.) 



FIG. 27. Stereognathus oolithicus. Frag- 

 ment of jaw with three teeth (a, 6, c), in 

 matrix ; from the Stonesfleld Slate. Natu- 

 ral size. (After Owen.) 



coupled with the circumstance that when the number of upper 

 incisors is reduced below three it is the second in place of the first 

 which becomes enlarged and opposed to the incisor of the lower 

 jaw, seems to prevent the acceptation of this view. Moreover, in 

 their peculiar structure the molars seem, on the whole, to make a 

 nearer approximation to the teeth of Ornithorhynchus than to any 

 other known mammal ; and it has accordingly been suggested that 

 the Multituberculata may really represent an order of Prototheria. 

 Some siapport is afforded to this suggestion by certain fragmentary 

 bones from the Cretaceous of the United States, which are regarded 



