UPPER PALEOZOIC FLORAS 325 
Alaskan where the same linear-lobed Sphenopteris forms are also 
present; the Nova Scotian affiliates more closely with the eastern. 
All the genera mingle in Arctic Europe and in Siberia, where Cyclo- 
stigma, probably of Arctic birth, has a good development. The 
Pocono flora may have connected by a more northern route with 
Europe, where Triphyllopteris sparingly mingles with the Horton 
Aneimites, which presumably migrated by the Northeast Arctic land 
bridge. It is possible, however, that some of the regional differences, 
especially differences in species, are due to lack of exact synchronism 
in the plant beds of these remote regions. 
Middle Mississippian flora——The basal Carboniferous floras are 
largely replaced in the middle Mississippian by a plant association 
which is more varied and of very different aspect. Where conditions 
were favorable for plant growth and preservation we find a flora 
essentially consisting of Rhacopteris (of Schimper, including Rhodea 
of authors), Cardiopteris, Asterocalamites (=Bornia), with Lepr- 
dodendron volkmannianum, and L. veltheimianum, accompanied by a 
gradually increasing group of Sphenopterids. 
Source and distribution—The middle Mississippian flora probably 
had its greatest development among the islands and estuaries of 
western Europe; at least it is best known in that region. From there 
it seems to have extended almost homogeneously to the eastward into 
Siberia and to the southeast, either through the Balkans, Persia, and 
the Himalayas (linking together its discovered occurrences), or pos- 
sibly by a more southern route, to South Africa and Australia where 
the flora was largely identical with that in Siberia. The flora at 
Cacheuta, in Argentina, which though small is mainly composed of 
European species, presumably traversed the same route as that later 
followed by the Gangamopteris flora—that is the “ Gondwana land.” 
The middle Mississippian flora of the Appalachian trough is but little 
known, and for the most part is unpublished. ‘Though less closely 
bound to that of Europe than are the corresponding Pennsylvanian 
floras its genera and a number of its species are present in the basins 
of Europe. I may add that the Megalopteris-bearing beds along the 
Mississippi River in Illinois, long ago credited to the Chester, are of 
upper Pottsville age. 
Moderate uniformity of climate-—The members of this flora do not 
