1893.] Succession of the Teeth in Mammals. 503 
Order of Appearance of the Cusps. The author observes 
that while in the lower jaw the order and appearance of the 
cusps is the same in the embryo as in the paleontological his- 
tory, in the upper jaw this ontogenesis is no longer parallel 
with phylogenesis, (as regards the upper teeth, this exception 
is confirmed by Röse). In fact the external cusps not only 
appear before the internal cusp, which paleontology shows 
to be more primitive but they assume the crescentic form 
earlier. In other words, their development is accelerated. 
The development of the premolars is also traced and 
it is interesting to recall the fact that in the paleontological 
history, the order of evolution of the cusps of the premolars 
is not the same as that of the molars. Taeker’s results there- 
fore show a parallelism between Ontogenesis and Phylogene- 
sis, in that he proves that the embryonic cusp order in the 
‘premolars is different from that in the molars, and is approxi- 
mately similar to that in the ancestral history as recently 
worked out by Scott. 
Upon the whole this parallelism between embryogenesis and 
palingenesis is most striking, and I think we can explain the 
exceptions which Taeker and Röse have shown to occur in the 
upper teeth both in the Primates and Ungulates, in the follow- 
ing manner: In the most primitive types of trituberculates, 
the protocone was the most prominent cusp in both jaws. 
This is seen in all the known Triassic and Jurassic trituber- 
culates without exception. During the Cretaceous period a 
change took place, in which the upper molars rapidly diverged 
in pattern from the lower molars. The lower were more con- 
servative, retaining the trigonid or triangle in its primitive 
proportions. The upper molars, on the other hand, must have 
undergone a marked change, consisting mainly in the depres- 
sion of the protocone below or to the level of the paracone and 
metacone, as seen in the primitive Carnivores, Creodonts, 
and Insectivores of the Puerco period. Thus we find both in 
the Ungulates and sub-Ungulates or early Primates, that in 
the upper molars the outer two cusps are slightly more ele- 
vated and decidedly more progressive in the acquisition of 
new forms than the older internal protocone. For example 
