DEVONIAN AND MISSISSIPPIAN FAUNAS 115 



north which had obtained during some of the earher Mississippian 

 periods. 



The faunas of the Chester beds have a certain individuaUty of 

 their own, ahhough the successive Umestone beds, in which the fossils 

 mostly occur, have not yet been faunally differentiated with any great 

 success. A conspicuous feature of the fauna is the presence of numer- 

 ous blastoids of the genus Pentremites, and bryozoans, especially of 

 the genus Archimedes. Among the brachiopods, especially, there is 

 some recurrence of species identical with, or closely aUied to, forms in 

 the Salem and Ste. Genevieve limestones, but this characteristic is not 

 limited alone to the brachiopods. 



In the typical portion of the Mississippi Valley Basin the Missis- 

 sippian period closes with the withdrawal of the Chester sea. Farther 

 to the southwest, in Arkansas, however, toward the more open sea, 

 it has been suggested by Ulrich' that similar faunas persisted into 

 beds which are really of Pennsylvanian age, under which interpreta- 

 tion the Une between the Mississippian and the Pennsylvanian, in that 

 region, would be somewhat arbitrarily drawn. It is not improbable 

 that the Arkansas beds are younger than any in the Mississippi \'alley, 

 yet that fact should not necessarily be considered as sufhcient basis 

 for referring them to the Pennsylvanian. The time boundary between 

 the two periods should be marked by the time of maximum withdrawal 

 of the sea or the subsequent readvance during which new sets of condi- 

 tions were introduced. 



MISSISSIPPIAN FAUNAS OF THE APPALACHIAN BASIN^" 



During Mississippian time the Cincinnati arch constituted a barrier 

 between the central Mississippian sea and the Appalachian basin, a 

 gulf which lay between this island and Appalachia. Into this basin 

 clastic sediments were being carried from the east, north, and west, so 

 that the pure limestones of the Mississippi Valley are absent, and the 

 faunas are neither so prolific nor so well differentiated. In this basin 

 the Mississippian formations are included within the Pocono and 

 Mauch Chunk formations of Leslie. The most definite point of faunal 

 contact between this basin and the Mississippi Valley Basin is found in 



1 Professional Paper, V. S. G. S., No. 24, p. 109. 



2 For a detailed description of the stratigraphy and correlation of the Mississippian 

 of the Appalachian Basin, see Stevenson, Bull. Geol. Soc. Am. XIV, 15-96. 



