210 
of the American monkeys in the union of 
the rami of the jaw at the symphysis, in the 
small size of the condyle, in the crowded 
condition of the teeth, and in the number 
of the incisors, canines and true molars 
which are also nearly alike in constitution. 
Northarctus possesses one more premolar 
and the others have a pair of fangs. The 
resemblance is so close that but little change 
would be necessary to evolve from the jaw 
and teeth of Northarctus that of a modern 
monkey. The same condition which would 
lead to the suppression of the first premo- 
lar, in continuance would reduce the fangs 
of the other premolars to a single one. 
This change with a concomitant shortening 
and increase in the depth of the jaw would 
give the characters of a living Cebus.” 
In studying the rich collections of this 
Primate material in the Yale Museum, I 
have been forcibly struck with the accuracy 
and wisdom of Leidy’s comparison. Not 
only does it appear to me that there is a 
decided resemblance between these extinct 
North American forms and the living South 
American Cebidz, but I am firmly of the 
Opinion that the one was the direct de- 
scendant of the other. This proposition I 
find is sustained by a number of trenchant 
osteological facts which I hope to fully 
elucidate in my forthcoming publication on 
the subject. 
While they may in general be said to be 
in a corresponding stage of evolution to 
that of the living Lemurs, they nevertheless 
belong to a separate and distinct phylum. 
This phylum detached itself from the main 
Primate stem far back in early Tertiary 
time. When the tropical forests receded 
to the southward of the land bridge which 
connected Asia with North America during 
practically the whole of the Tertiary, the 
interchange of species which were depend- 
ent upon this tropical vegetation was no 
longer possible, and at this point began the 
devclopment of the two phyla, one of which 
SCIENCE. 
[N.S. Vou. XIII. No. 319. 
terminated in the Old World monkeys and 
man and the other the flat-nosed monkeys 
of the New World. 
In the earlier half of the Middle Eocene 
or Wasatch these monkeys were abundant 
in northern Wyoming. During the deposi- 
tion of the Bridger beds or first stage of 
the Upper Eocene, we find them in southern 
Wyoming, where they may be said to have 
reached their culmination, so far, at least, 
as numbers and variety are concerned. We 
are at liberty to infer from much collateral 
evidence that this region was at that time 
intensely tropical and was highly suited to 
their manner of life, since some of the more 
common species are represented by remains 
of thousands of individuals in the collec- 
tions. Towards the close of the Bridger, 
however, premonitions of the coming 
changes began to make themselves felt, 
probably first in the altered character of 
the vegetation. These were of such a nature 
as to cause another retreat to the south- 
ward, which must have been more rapid, 
since in the succeeding Unita or uppermost 
Eocene stage we find but a few stragglers 
of the hardier and more abundant species 
left behind. Such, however, soon disap- . 
pear, aud with this record, so far, at least, 
as our knowledge at present extends, their 
history on the North American continent 
closes. It isa highly significant fact that 
in the succeeding Oligocene and Miocene 
deposits the remains of monkeys are wholly 
absent, in the northern latitudes, at least, 
where they were formerly so abundant. 
The only conclusion we can draw from this 
is that the tropical vegetation had receded 
to the south, and taken with it all the in- 
habitants dependent upon it. 
We next hear of these Primates in the 
Miocene beds of Patagonia, showing clearly 
that they had not only reached the South 
American continent at that time in their 
southward peregrinations, but that they had 
spread themselves quite to its extreme south- 
