ABSTRACT: Several Late Pennsylvanian (Early Missourian) age limestone beds exposed in isolated outcrops in the Trinity River Valley in southwestern Wise County and named by Böse (1917), and Scott and Armstrong (1932) are assigned to the Palo Pinto Formation (lower part of the Canyon Group). Clearly designated type localities or type sections were not given for some limestone beds named in these two previous studies. These limestone units in southwest Wise County occur in coherent stratigraphic sequences in three key areas: along Dry Creek, around Martin Lake, and along Boons Creek. Original type localities have been located and suitable reference sections have been established for these limestone beds. The Canyon Group was established for outcrops of strata in the Brazos River Valley. Correlation of strata between the type area and strata in the Trinity River Valley is based on stratigraphic position, textural characteristics of the limestone, and similarities in fl ora and fauna, including the presence of distinctive algae and fusulinacean species of Triticites and Nankinella. Equivalence of several of the units named by Scott and Armstrong (1932) is established: the Boone Creek Limestone and Hudson Bridge Limestone; the Sanders Bridge Limestone and unnamed (Yl4) limestone; and the Martin Lake Limestone and Balsora Limestone. The names fi rst used in the literature and retained here are the Boone Creek Limestone, Sanders Bridge Limestone, and the Martin Lake Limestone. Two additional limestone units present between the Martin Lake Limestone and the Willow Point Limestone are formally designated here as the Bridgeport Road Limestone and the Kirkman Limestone. The fi ndings of this investigation follow the Canyon Group nomenclature established by Laury (1962). The two new limestone units, along with the Bridgeport Coal and the Willow Point Limestone, are placed in the Posideon Formation, with the Willow Point Limestone forming the top member. In the Posideon Formation, the Bridgeport Road Limestone may correlate to one of the lower limestone units within the Pp1 shale of Laury (1962) in the type area. The time-synchronous overlying conodont rich core shale has been correlated previously to the Pp1 shale in the type area. The Kirkman Limestone is correlated to the Pp2 limestone, and the Willow Point Limestone to the Wiles Limestone, both correlations based on the presence of overlying time-synchronous core shale in both river valleys. The Sanders Bridge Limestone is correlated to the middle of the Palo Pinto Formation in the type area in the Brazos River Valley.