506 The American Naturalist. [June, 
distinct cones, these cones should certainly appear sub-equal 
from the first, and they should present a division extending to 
the base and indicated in the fang. From the fact that 
Kükenthal does not mention the above genera, we infer that 
his attention has not been called to them. In one of his 
latest addresses he speaks as if the multituberculates were the 
only Triassic mammals. We know that the trituberculates 
were as old or older than the multituberculates, and reasoning 
by analogy, I have attempted to show elsewhere that the 
multituberculates will be found to have a trituberculate 
origin. 
As I have lately criticised this theory in some detail in the 
“Anatomischer Anzeiger,” I here refer to the well-known 
evidence which renders the Kiikenthal-Rése hypothesis both 
superfluous and untenable; when we find in the series such 
as is represented by the genera Dromotherium, Microconodon, 
Spalacotherium, Amphit , Miacis, exhibiting in the lower 
jaw all the stages between the sectorial and bunodont crown, 
and when in the early Ungulates we are able to trace one by 
one the successive additions of cusps to the bunodont molar, 
often in the center of the crown, it certainly is not necessary to 
attempt to establish any new hypothesis as to cusp origin. 
SUMMARY OF RESULTS. 
The general bearing of these researches upon the stem forms 
of the pro-mammalia is extremely important. To summarize, 
it is rendered probable that the earliest forms of mammals 
were homodont and had two or more series of successional 
teeth. Then within the mammalian stem the teeth were 
differentiated, and there arose a great heterodont group with 
teeth at least of three kinds—incisors, premolars and molars, 
all successional. From the most anterior premolar arose the 
canine. Then came the division between the Marsupials and 
the Placentals, the former tending to suppress the develop 
ment of the second series of teeth, the latter retaining the 
second series as far back as the first molar. 3 
We can see an obvious advantage in the line of succession 
