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Trilobites collected during the past twenty years from the Morgan Creek, Point Peak, and San Saba members of the Wilberns Formation constitute the material basis for this paleontologic and statistical investigation. Eighty-nine species assigned to forty-five genera belong to zones of the upper Franconian and Trempealeauan Stages of the Upper Cambrian Croixan Series. New zonal names are proposed in the interest of a regionally applicable nomenclature. Although none of the zonal nomenclature is identical to that of the Cambrian Correlation Chart, the four zones recognized in central Texas are equivalent to the eight highest zones of the Chart. Stratigraphically lowest is the Franconian Taenicephalus zone, with a locally recognized Parabolinoides subzone at its base; this is equivalent to the Conaspis zone of the Correlation Chart. The Franconian Idahoia zone, with a locally recognized Idahoia lirae subzone at its base, is equivalent to the Ptychaspis subzone of the Ptychaspis-Prosaukia zone of the Correlation Chart. The sparsely fossiliferous Ellipsocephaloides zone corresponds to the Prosaukia subzone of the Ptychaspis-Prosaukia zone of the Chart. Almost two-thirds of the trilobite species occur in the Trempealeauan Saukia zone, which corresponds to the five highest zones of the Correlation Chart; local subzones, in ascending order, are the Saukiella pyrene subzone, the Saukiella junia subzone, the Saukiella serotina subzone, and the Corbinia apopsis subzone. The succession of ptychoparioid trilobite faunas contained within these zones make up the Ptychaspid biomere. The base of the biomere is at the base of the Taenicephalus zone; the top coincides with the lowest occurrence of the Ordovician trilobite fauna. Trilobite families that characterize the Ptychaspid biomere are the Ptychaspididae and the Parabolinoididae. Regression analyses of range data for all Saukia-zone taxa were used to compile a quantitative range chart. Differences between the quantitative chart and my intuitive range chart were not significant. Systematic descriptions include new species of Conaspis, Idiomesus, Euptychaspis, Keithiella, Saukiella, Prosaukia, Calvinella, and Westonaspis?; and one new variety of Orygmaspis. / text

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