284 STUART WELLER 
etc., suggest also a relationship with the Salem limestone fauna of the 
Mississippi Valley. The conditions are therefore similar to those in 
the Madison limestone of the North, and the interpretation of the faunal 
relations in that region can doubtless be extended to the more southern 
area. 
MISSISSIPPIAN FAUNAS OF THE WESTERN CONTINENTAL PROVINCE 
For a knowledge of the Mississippian faunas of the Great Basin 
region we are especially indebted to Walcott, who has described them 
from the Eureka district of Nevada.t| The faunas occur at various 
horizons through a series of “‘ Lower Carboniferous” limestones 3,800 
feet in thickness, and are most remarkable from the fact that there is 
a general mingling of forms which, if found in the Mississippi Valley, 
would be considered as characteristic either of the Devonian, the 
Mississippian, or the Pennsylvanian. There is, however, a notable 
absence of the more conspicuous elements of the Mississippian faunas 
of the Mississippi Valley, such as the crinoids of the Osage faunas, 
the large Spirifers of the S. striatus type, the Archimedes and Pentre- 
mites of the Chester faunas, etc. None of the specialized Mississippi 
Valley faunas can be recognized. This basin must have been isolated, 
during Mississippian time, both from the Mississippi Valley and from 
the Rocky Mountain basins. The one point of faunal contact between 
the Great Basin and the Mississippi Valley is found in the presence of 
several of the peculiar Great Basin forms in the fauna of the Spring 
Creek limestone of northern Arkansas, among which Rhynchonella 
eurekensis and Leiorhynchus quadricostatus are perhaps the most 
notable. The age of the Spring Creek limestone is believed to be very 
close to that of the Ste. Genevieve limestone, at which time, perhaps, 
the Mississippian Sea had its greatest extension in the East. With this 
expansion of the sea there would seem to have been established a 
brief communication with the Great Basin region, of such a nature as 
to allow a group of these peculiar forms to migrate at least as far east 
as northern Arkansas. It is apparently impossible to correlate this 
incursion in the Great Basin, however, perhaps because of our 
imperfect knowledge, because the most notable of the immigrant 
species, R. eurekensis, has a long range in the Great Basin beds. 
t Paleontology of the Eureka District, Monog., U.S. G.S., Vol. VIII. - 
