110 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. 2+). 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 (ig. 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 
-—The right ramus of the mandible of Plagicaulae those forms tty which 
deklesi; trom the Purbeck of Swanage, Twiee natural size. these teeth are known, 
i, Incisor; ne, molar ; b, coronvid process ; c, condyle. (Atte while there are, at least 
aii usually, only two in 
those of the lower jaw. In other cases the premolars ave of a 
secant type, with a highly convex cutting-edge, and usually cither 
serrated or obliquely grooved (Figs. 25, 26). 9 From a certain 
resemblance between these secant premolars and those of some of 
the smaller Jacropodid it was at one time considered that we had 
in these mammals representatives of Diprotodont Marsupials. The 
great difference in the strueture of the molar teeth of these forms, 
Fic. 26,—The imperfect right ramus of the Fra. * Sterecgnathus odlithicus, Frag: 
mandible of Migiaulae minor; from Swanage. ment of jaw with three teeth (a, d, e), it 
Four times natural size. p, Premolars ym, matrix; from the Stonestield Slate. Natue 
molars. (After Lyall.) ral size. (After Owen.) 
coupled with the circumstance that when the number of upper 
incisors is reduced helow three it is the second in place of the tirst 
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 sueeested that 
the Multituberculata: may veally represent an order of Prototheria. 
Some support is afforded to this sugeestion by certain fragmentary 
bones from the Cretaceous of the United States, which are rewarded 
