K1NE TO GENESIS. 325 



movement of the inferior molars on the superior in 

 Plagiaulacidae. This was no doubt the case in the 

 oth^r families named. The molar teeth of the lower 

 types, as Tritylodon from the Trias, 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 the three in the upper jaw, moving 

 in the grooves between the latter, while the three se- 

 ries 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. 93). The trituration 

 was probably the same in Tri- 

 tylodon, but in Polymastodon 

 the increased thickening of 

 the tubercles prevented their 

 interlocking action in masti- 

 cation. In this genus the tu- 

 bercles Slid OVer each Other, f\^.g^. A, Meniscoessus conquis- 



and truncated the apices ^ cope, last two superior molars. 



from the Laramie of Wyoming, 

 Until in Old Specimens they twice natural size. B, Meniscoessus, 



were entirely worn away. In second species ' from Osborn ' 

 Meniscoessus (Fig. 94) 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 angles in the direction of thrust, converting 

 their transverse sections from circles to crescents. As 

 the thrust is in the Multituberculata longitudinal, the 

 crescents are transverse to the axis of the jaw. In the 

 selenodont Artiodactyla, where the thrust is transverse 

 to the line of the jaw, the crescents are longitudinal. 

 That similar effects should accompany similar move- 



