1894. | NEW YORK ACADEMY OF SCIENCES. 235 
The differences between these two groups consist mainly in the 
lower state of evolution and apparent incapacity for higher devel- 
opment of the former and the higher state of evolution with ca- 
pacity for rapid development of the latter. The value of this dis- 
tinction consists in the fact that the Mesoplacentals evolved and 
diverged in North America and undoubtly in Europe, during 
Mesozoic times, in the Jurassic, Cretaceous and Lower Tertiary. 
Careful studies of the Upper Cretaceous mammals show that they 
probably had already diverged into Ungulate and Unguiculate, 
Carnivorous and Insectivorous types. This functional diver- 
gence reached its climax in the Puerco, which Professor Cope 
is inclined to consider as the summit of the Cretaceous instead 
of the base of the Eocene. Here these mammals exhibited 
their greatest variety, and are found to be characterized by 
plantigrade feet and tritubercular teeth. One especial feature 
of all the hoofed types is that they develop their molar type, 
whether bunodont, selenodont or lophodont, upon the triangu- 
lar plan. We may consider the Dinocerata, Creodonta and 
Tillodonta as spurs of this great Mesoplacental radiation, and 
we find the key to the extinction of the Mesoplacentals in their 
stationary brain develop and comparatively defective tooth and 
foot structure. 
It is important to emphasize the fact that we have not as yet 
connected any of these Mesoplacentals directly by lineal de- 
scent with the Cenoplacentals, excepting perhaps Huprotogonia, 
a supposed ancestor of the Perissodactyla (Schlosser) and 
Protogonodon, a supposed ancestor of the Artiodactyla (Earle). 
But even when such threads of lineal descent are traced by fu- 
ture research, the fact remains that the great group of Meso- 
placentals as such became extinct ; that the first attempt at wide 
functional radiation of the mammals failed, and that from some 
comparatively unspecialized spurs of the dying Mesoplacental 
group, a new functual radiation began, reaching its climax in 
the Cenoplacentals of the Miocene period, but since then de- 
clining. 
There exists therefore a threefold parallelism among the mam- 
mals, of similar adaptive structive developed independently 
among the Marsupials, Mesoplacentals and the Cenoplacentals. 
Very probably when the Monotremes are more fnlly known, we 
shall also find that they exhibited their own functional radiation 
into a variety of types, of which the Multituberculates with their 
strong Rodent analogies, the Ornithorhynchide and Echidnide 
are the only present ones known. 
The Mesoplacentals are not defined as a homogeneous group ; 
they are very heterogeneous ; what unites them is the incapacity 
