: OC. A. White—Fossil Faunas and Floras. 369 
aid of known stratigraphical continuity. As my present object 
is to discuss certain of the faunas of the preceding formations 
with relation to the order in which the latter were deposited, 
those later Tertiary deposits will not be further considered on 
this occasion. ; 
What we now know of the order of succession of the verte- 
brate faunas which have inhabited that western region, so far 
as that knowledge is necessary to the present discussion, may 
be briefly summed up as follows: Dinosaurian life continued 
until the close of the Laramie period and then suddenly ceased. 
The Puerco mammalian fauna was suddenly introduced within 
the limited area where its remains have been found, and as 
suddenly extinguished there about the close of the Laramie 
period or the beginning of the Wasatch. The Wasatch mam- 
malian fauna was suddenly introduced in the region. under dis- 
cussion at the close of the Laramie period and coincident, or 
nearly so, with the extinction of the dinosaurs or, at latest, 
with that of the Puerco mammals. 
The dinosaurs of the Laramie period were evidently not 
materially different in general character from those of the pre- 
ceding periods; and there is no reason to doubt that they were 
direct descendants of the latter. They found a congenial habi- 
tat upon the- borders of a great inland sea in the deposits of 
which their remains are found, and until the Puerco mamma- 
han fauna was introduced into a part of the region which they 
occupied. They also appear to have continued their existence 
in at least certain parts of their former habitat after the intro- 
duction of the Puerco fauna, and until the introduction of the 
Wasatch mammalian fauna into the same region. We have no 
positive evidence that the Laramie dinosaurs continued their 
existence in direct association with either the Puerco or 
Wasatch mammalian faunas, because no Dinosaurian remains 
have been found associated with those of either of the faunas 
named. The probabilities are that the dinosaurs met their 
extinction in the struggle for existence which ensued upon 
the introduction of one or both the mammalian faunas just 
mentioned. 
Now it seems necessary to infer that these changes in the 
character and continuity of vertebrate life were mainly due to 
important changes which from time to time took place in the 
physical conditions of the continent. And yet it is evident 
that those physical changes were not sufficient to interrupt the 
continuity of either the invertebrate or plant life in the same 
region, nor to wholly interrupt the continuity of sedimentation 
in the intra-continental waters. In short it is quite evident 
that the land vertebrate life of North America has been sub- 
ject to certain influences which effected great changes in, and 
