12 Origin of the Dentition of the Rodentia. 
NOTE ON THE MARSUPIALIA MULTITUBERCULATA.—The struc- 
ture of the dentition of this suborder is in many respects like that 
of the Rodentia in the known forms. The incisors in the Plagiau- 
lacidee, Chirogidee and Polymastodontide have structure and func- 
tions generally similar to those of the Rodentia. The result in 
the form and function of the molar dentition has been similar to 
that observed in the Rodentia. The postglenoid process is prob- 
ably absent in these animals; in any case the mandible, or condyle, 
is rounded and is not transverse, Professor H. F. Osborn has pointed 
out to me that mastication was 
performed by a fore and aft 
movement of the inferior mo- 
lars on the superior, in Plagiau- 
lacidee. This was no doubt the 
case in the other families named. 
The resulting structure of the 
crowns is, however, different, 
and needs explanation. The 
molar teeth present conical 
tubercles in longitudinal series, 
two in the lower, and three in 
the upper jaw. The two series 
of the lower jaw alternate with 
Fie. 9. Marsupialia erly ai eng sewed : 3 
pig.. a, Otenacodon serratus Marsh, $ from the three in the upper jaw, 
niscoëssus cong : s 
three-halves sea dis bo size; A go aan per or molar’ moving ın the grooves between 
ge, humeral condyles molar, Fig > 
d, Stereognathus oiliticus S Owen 3. ft ł, from Owen: the latter, while the three 
c, Tritylodon longaevus Owen 3, from Owen. + series of the upper molars 
reciprocally embrace the two of the lower molars. This is 
demonstrated by the mutual wear of the tubercles seen in 
Ptilodus and Chirox (Fig. 7). The trituration was probably the 
same in Tritylodon, but in Polymastodon the increased thickening 
of the tubercles prevented their interlocking action in mastication. 
In this genus the tubercles slid over each other, and truncated the 
apices until in old specimens they were entirely worn away (Fig. 
8 ce). In Meniscoéssus and Stereognathus we have an interesting 
illustration of the effect of the action of cusps on each other when 
under prolonged mutual lateral thrust. Their external sides have 
been drawn out into long angles in the direction of thrust, con- 
verting their transverse sections from circles to crescents. As the 
thrust is in the longitudinal Multituberculata, the crescents are 
