292 EEPORT OF THE AMERICAN COMMITTEE. 
EqiiidfB, Proboscidia, Cosoryx, Glyptodontidee, and Hystricidse. 
Absence of Tragulidse, Oreodontidre (with very few exceptions), 
Poebrotheriidse, Elotheriidse, and Nimravidse. 
This formation has a wide extent tliroughout North America. 
The larsrest area overlies the Whiteriver beds in Nebraska, 
Wyoming and Colorado, extending south and east of that for- 
mation into Kansas, where it rests on the Cretacic. There is 
a second area in Northern Central New Mexico, and one per- 
haps in Southern New Mexico, extending from the Rio Grande 
to near the Arizona border. There is another tract in Wash- 
ington County, Texas ; and yet another in Mexico, on the boun- 
daries of the States of Hidalgo and Vera Cruz. According to 
King its thickness in Wyoming reaches two thousand feet, but 
it thins out gradually to the eastward, so as to have a thickness 
on the White River of about one hundred and fifty feet, accord- 
ing to Hayden. 
This formation was referred to the Pliocene series by King and 
Hayden, and I have called it Upper Miocene. The latter view 
is supported by the presence of the following European Miocene 
genera and species : Cosoryx, Blastomeryx ; Castor div. Steneo- 
fiber ; Mastodon {Tetrahelodon) angustuJens. The remaining 
Oreodontidse (Merychyus), give it a facies older than Pliocene. 
This series has received the name of Niobrara from Marsh, 
a term previously applied to a division of the Cretacic. It in- 
cludes the Humboldt, and probably the North Park formations 
of King. 
PLIOCENE. 
Under this head I include everything between the Miocene and 
the glacial epoch. It includes the following divisions. Two of 
them are consecutive in time, viz. : 
Equusbeds. 
Idaho. 
Two others are probably contemporary with one or both of 
the preceding, so that the names have only a provisional utility. 
Megalonyx beds. 
Truckee. 
Idaho. — Present: Mammals, Camelidee, Equus exoehus ; 
Fishes, Cobitidse, Percidse, Silurida, Raiidse, Mylocyprinus 
(Cyprinidffi), and peculiar species of existing genera of Cottidse, 
