DEVONIAN AND MISSISSIPPI AN FAUNAS 119 



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 theNorth,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.^ 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 qiiadricostatus 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. 



I Paleontology 0} the Eureka District, Monog., U. S. G. S., Vol. VIII. 



