694 The Tertiary Marsupialia. [July, 
firms the theory of derivation from complex premolars, and we 
observe in the later form, Thylacoleo, a simplification of the true 
molars‘also.. The molariform fourth premolars in the existing 
Macropodidz confirm this view. In 
order to connect these latter with the 
ancestral form of the Plagiaulacida, 
on a former occasion, I posited a 
theoretical form which should com- 
bine three of the cutting premolars 
of the extinct family with the molar- 
iform fourth premolar of the Macro- 
podidæ. This I named Tritomodon. 
The discovery of Tritylodon hasadded 
confirmation to this view, at least as re- 
Bs Pat vards the prior existence of the molari- 
Fic. 8. — Péilodus medievus form premolars. The Polymastodot- 
Cope, left mandibular ramus, i ‘ the 
nat. Size; from the Upper Puerco tidz were probably derived from 
beds of New Mexico. Fig. e Tritylodontida by the usual process 
superior view, 3 nat. size. Orig. of reduction of number of teeth ; 
Miner "Pen PN a specialization of those that eer? 
in charge. 3 Thylacoleo? must be regarded as 
type of a family distinct from the Plagiaulacidz, since it has but 
one true molar tooth in the upper jaw. The composition of that 
tooth is unknown, so that it is not certain whether the anit 
Thylacoleontidz must be placed in the Multituberculata o - 
cophaga. That it is a direct descendant of the Plagiaulacid 
think there is no doubt. The following phylogenetic scheme K 
similar to'one I published in the NaTuratist, 1882, p. 525 b ) 
some addition, and the removal of Polymastodon (Cato: 
from the Plagiaulacide : 
1 NATURALIST, 1882, p. 5216 
* Owen, Quar. Journ. Geol. Society, London, 1883» 
mes 
