270 COPE. 
[VoL. III. 
Figure 92.— Polymastodon tatensis Cope, jaws two-thirds nat. size; from Puerco 
bed of New Mexico. Figs. a, d, lower jaw; ¢, upper jaw; original. 
Figure 93. — Monotremata Multitubercu- 
lata. Fig. a, Ctenacodon serratus Marsh; 
three times nat. size; from Marsh: d, 7, 
Meniscoéssus conguistus Cope, three halves 
nat. size: d, superior molar; ¢7, humeral con- 
dyles; #4? premolar. Fig. 6, Stereognathus 
obliticus Owen; three times nat. size; from 
Owen; c, Tritylodon longevus Owen; three 
times nat. size; from Owen. 
tubercles in longitudinal se- 
ries, 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 series of the 
upper molars reciprocally em- 
brace the two of the lower 
molars. This is demonstrated 
by the mutual wear of the 
tubercles seen in Ptilodus 
and ‘(Chiroxm (Pic: *o1) “he 
trituration was probably the 
same in Tritylodon, but in 
Polymastodon the increased 
thickening of the tubercles 
prevented their interlocking 
action in mastication. In this 
