The Middle Ordovician Simpson Group of southern Oklahoma consists of an exceptionally continuous record of carbonates, shales and quartz sandstones which accumulated in a shallow shelf environment. Vertical changes in the distribution of trilobite genera from these rocks can reasonably be interpreted as due to age rather than the influence of various environments or faunal provinces. The affinities of the lower Simpson trilobite faunas are with inner shelf faunas of trilobite zones M and N known from Utah and eastern Nevada and, to a lesser degree, with early Middle Ordovician faunas of Quebec and Newfoundland. Middle Simpson (McLish and Tulip Creek) Formations reflect slightly higher energy conditions and very few well-preserved trilobites. Those few genera present (for example, Pliomerops and Eorobergia) are more typical of the Chazy Group and correlatives from eastern North America. The higher Bromide Formation, also of shelf origin, displays a distinctly eastern fauna, very similar to elements found in the Chazy Group and the presumably younger Edinburg Group of the east. Although many of Cooper's (1956) Middle Ordovician correlations can be understood, the stage names proposed at that time overlap in some cases and lack proper biostratigraphic definition in others. Abandonment of these stage names is urged until description of Middle Ordovician conodont, graptolite and shelly faunas now underway is nearer completion.
