Trilobites collected during the past 20 years from the Morgan Creek, Point Peak, and San Saba Members of the Wilberns Formation comprise 89 species assigned to 45 genera belonging 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 1944 Cambrian Correlation Chart of Howell et al., the four zones recognized in central Texas are equivalent to the eight highest zones on the Chart. Stratigraphically lowest is the Franconian Taenicephalus Zone, with a locally recognized Parabolinoides Subzone at its base; this zone 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 on the Chart. Almost two-thirds of the trilobite species described 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 junta Subzone, the Saukiella serotina Subzone, and the Corbinia apopsis Subzone.

The succession of ptychoparioid trilobite faunas within these zones characterizes the Ptychaspid Biomere. The base of the biomere is at the base of the Taenicephalus Zone; the top coincides with the lowest occurrence of an Ordovician trilobite fauna. Trilobite families that characterize the Ptychaspid Biomere are the Ptychaspididae and the Parabolinoididae.

Systematic descriptions include two new subfamilies, Drumaspidinae and Ptychaspidinae, and eight new species, Conaspis leptoholcis, Idiomesus infimus, Euptychaspis frontalis, Keithiella scapane, Saukiella serotina, Prosaukia remora, Calvinella prethoparia, and Westonaspis? texana.