OC. A. White—Fossil Faunas and Floras. 367 
of these faunas is very great, even when they are found to 
oceupy stratigraphical horizons so closely approximating each 
other as to indicate that they were not separated by any con- 
siderable interval of time. ‘his condition of things is suffi- 
ciently perplexing from a purely zoological point of view, but 
it assumes an additionally serious aspect when these sudden 
appearances and disappearances of peculiar faunas are used as 
indices of the exact limits of geological epochs. 
While it is clear that many faunas may have been rapidly 
extinguished by sudden changes in their environment, we cannot 
admit that any have been introduced into a given area with 
anything like the suddenness that is indicated for certain of 
the faunas presently to be mentioned, except by some physical 
change that should cause the enlargement of the area previ- 
ously occupied by such a fauna, or its transference from one 
region to another. That is, it is assumed that in case a distinct 
-fauna has been suddenly introduced into any given region, it 
has been by transference from some other region which it had 
previously occupied, and where its evolution had been accom- 
plished. 
The first of the. vertebrate fauna which I shall mention in 
this connection is that of the Dinosauria of the Laramie Group. 
This is the latest dinosaurian fauna which is known to have 
existed upon the North American continent, and it is probably 
the very latest that existed upon the earth. Its remains are. 
comparatively rare, although large and characteristic forms 
existed until the close of the Laramie period. ‘These dino- 
saurian remains are the only fossils yet found in the Laramie 
Group upon which has been based the claim that it is of Cre- 
taceous age; and the character of the plant and invertebrate 
remains in the same formation is such that but for the presence 
of the dinosaurs no one would have thought of referring it to 
any Mesozoic period. The few remains of dinosaurs that have 
been found in the Laramie Group range up to its uppermost 
strata, and the extinction of those forms seems to have been 
simultaneous with that of the brackish water mollusca whose 
remains characterize the Laramie Group, so far as its inverte- 
brate fauna is concerned. 
Professor Cope has published an important mammalian fauna 
from certain strata in New Mexico, which he designates as the 
Puerco Group, the position of which is between characteristic 
Laramie strata beneath, and equally characteristic Wasatch 
strata above. These Puerco strata were originally referred to 
the Wasatch Group and, as they are also of fresh water origin, 
and not widely different in lithological character from those of 
the Wasatch, which rest conformably upon them, it is not 
probable that any other designation would have been given 
