280 STUART WELLER 
north which had obtained during some of the earlier Mississippian 
periods. 
The faunas of the Chester beds have a certain individuality of 
their own, although the successive limestone 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 allied 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 line 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 Valley, 
yet that fact should not necessarily be considered as sufficient 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 
t Professional Paper, U.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. 
