UPPER CARBONIFEROUS 309 
Upper Carboniferous faunas of the continental basin do not show 
differentiation during this long interval, for the Pottsville group has 
a distinct fauna and appreciable changes occur in the later Pennsyl- 
vanian. But the changes are by no means so marked as one would be 
led to expect from the thickness of the strata involved, the extent of 
the territory they cover, and the varying conditions of the time and 
the place. The truth of this statement will be appreciated upon a 
consideration of the Mississippian series of the Upper Mississippi 
Valley and the subdivisions which have been established in it. 
When we speak of the variety of conditions under which the 
sediments in question were laid down, and remember that they include 
the great coal deposits of this era, we are led to inquire whether the 
faunas are marine or fresh water, or perhaps both, with the important 
difference in facies which such difference in habitat would doubtless 
entail. Fresh-water faunas, or at least fresh-water genera and species, 
have been recognized elsewhere in the Carboniferous, notably in the 
Coal Measures of England and the Permian of Russia. In North 
America, although we have a facies which appears to be non-marine, 
_ there are no Carboniferous faunas which in my belief can be called 
fresh water. ‘The facies in question recurs frequently, particularly in 
the Appalachian region, and manifests little change in its general 
aspect, although appearing at widely different horizons in the Pennsyl- 
vanian. It is very restricted in variety though often abounding in 
individuals. A mollusk probably identifiable as Nazadites elongatus 
Dawson is a characteristic feature. Ostracods are also abundant, and 
the large bivalve Crustaceans, Estheria and Leaia, sometimes occur. 
Spirorbis is another type frequently met with, while fish scales, frag- 
ments of Limuloid Crustaceans, and wings of insects are rare. Usually 
this peculiar assemblage of forms is associated with abundant coal 
plants. 
The genus Nazadites was described by Dawson from the Nova 
Scotia Coal Measures, in which it occurs with a fauna similar to that 
sketched above. Dawson regarded the sediments and faunas as 
representing fresh-water conditions, and considered Naiadites to be 
related to the Naiads of our fresh-water lakes and rivers. 
The fresh-water mollusks of the English Carboniferous were 
included by Dr. Wheelton Hind under the three genera, Anthraco plera, 
