1000 The American Naturalist. [December, 
Phascolotherium and the later Triconodon. It is a very signifi- 
cant fact that this type dies out in the Upper Jurassic. It is 
true we find many more recent “ triconodont ” teeth, the lower 
molar of Mesonyx for example, which are positively known to 
be of tritubercular origin. Richard Owen compared the lower 
molars of Thylacinus with those of Triconodon, but we have 
found that what appeared to him to be similar cusps are not 
really homologous. Thus while it is possible that the ances- 
tors of some of the modern haplodont and triconodont mam- 
mals never reached the tritubercular stage, it is by no means 
a settled fact. On the other hand, excepting the isolated 
group of Multituberculates and the single genus Dicrocynodon 
Marsh, the molars of every known fossil mammal from the close of 
the Lower Cretaceous until the close of the Eocene period bear the tri- 
tubercular stamp. 
This would appear to support the generalization that all 
mammals passed through the third primary or tritubercular 
stage, yet it must be borne in mind that all our evidence is 
derived from inhabitants of fresh water basins, and that the 
persistent haplodont and triconodont types may have been 
living contemporaneously in the seas. 
But the Multituberculates and Monotremes, were they tri- 
tubercular in origin? The teeth of Ornithorhynchus are so 
degenerate and irregular that many features of primitive form 
may be lost; they may quite as readily be interpreted as 
tritubercular as multitubercular, especially in the embryonic 
stage as described by Poulton. 
It is not difficult however to establish the principle that a 
true multitubercular tooth may spring from a tritubercular 
tooth. As pointed out elsewhere, my friend, Prof. J. A. Allen, 
directed my attention to the “multituberculate” rodents. A 
comparison of Mus, Dipodomys and Perognathus beautifully 
illustrates the stages between “trituberculy ” and “ multitu- 
berculy ” in living types. The three rows containing twelve 
tubercles in the later genus are derived respectively from the 
“ external,” “intermediate” and “internal ” cusps of a sexitu- 
bercular bunodont type similar to the Hyracotherium molar on 
a small scale. The additional cusps are successively added to 
