The conodont fauna of the Glen Dean formation (Chester, Mississippian) is known to be uniform in composition and distribution throughout the Illinois basin. However, virtually no conodont occurrences have been described from a geosynclinal environment, and no study has been made to relate shelf and geosynclinal occurrences. Such a study is the primary purpose of this paper. Although approximately 1,850 specimens were recovered from the 6 sections sampled, less than one-third of them are generically identifiable, exclusive of fragments of the long ranging genus Hindeodella. Twenty-seven named species distributed among 13 genera, including Hindeodella, are represented. The most abundant species is Cavusgnathus unicornis Youngquist & Miller. The second most common is Neoprioniodus scitulus (Branson & Mehl). The distribution of species within the area studied is quite uniform, but abundance of the conodonts varies markedly in differing lithologies and with the thickness of the sections involved. It is suggested that the difference in the faunas the present study, the Illinois basin Glen Dean, the Gnathodus bilineatus zone of the Barnett formation of Texas, and the Delaware Creek member of the Caney shale is primarily one of geographic province rather than a time disparity. The Illinois basin represents one faunal province, the indicated portions of the Barnett and Caney represent another, and the area of this study shows commingling of the two. There is no evidence of any middle Mississippian element in the parts of either the Caney or Barnett as mentioned above. Streptognathodus, which may be as old as very uppermost Chester but is commonly considered Pennsylvanian, is present in the upper part of the Sand Branch member of the Caney shale and beds considered to belong with the Barnett. Accordingly, the Delaware Creek member of the Caney and the Gnathodus bilineatus zone of the Barnett must be considered of Chester age, with the possibility that they may also represent deposition into the Pennsylvanian.

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