Fauna of the Morrow Group 
67 
and elastics of the eastern portion. Does the Morrow of Okla- 
homa represent there the entire series as developed in Arkansas, 
or is it due to the westward spread of the Morrow seas during 
one of the three intervals of deeper water implied by the forma- 
tion of the limestone lenses in the midst of the clastic deposits 
in the eastern basin which presumably was adjacent to the 
higher lands from which the sediments were derived? 
From the undifferentiated Morrow of Oklahoma ninety-one 
species have been identified and all but nineteen of them are 
forms which occur also in the various horizons of the group in 
Arkansas. The relations of this fauna to the three Arkansas 
faunules is shown in column 4 of Table 11. The evidence points to 
the equivalence of the Morrow limestone of Oklahoma and the 
Brentwood limestone member of the Morrow group in Arkansas. 
Analysis of the Morrow Fauna 
A critical examination of the Morrow fauna makes evident 
its position as transitional between the faunas which have been 
considered typical of the later Mississippian and of the earlier 
Pennsylvanian times. The reference of the Morrow group to 
the early Pottsville, which has been made on the basis of the 
flora occurring in the Bloyd shale between the Brentwood and 
Kessler members, appears to be verified by the faunal evidence 
from the associated limestones. The fauna is essentially a 
mingling of two assemblages of animals, one group composed 
of forms of a Mississippian type which had persisted in basins 
to the south to which they had retreated with the withdrawal 
of the Chester seas from the Mississippi valley region, and the 
other including those forms which are typical of the Pennsyl- 
vanian period and either had evolved in the southern basin after 
the Chester retreat or had invaded the Morrow seas at or before 
the time of advance into the Arkansas-Oklahoma locality. The 
former may be conveniently referred to as the residual element 
and the latter as the proemial^ element. 
Among the corals, the subclass Tetracoralla is represented by 
four species belonging to four of the more common Carbonifer- 
*J. M. Clarke has applied the term “proemial’’ to ‘‘an introductory 
fauna which heralds the incoming of a new organic association and passes 
gradually, without interval or interruption, into that culminant assem- 
blage.”2o 
