This paper is the first part of an Upper Cambrian faunal study designed to supplement Deiss’ stratigraphic work on the revision of the type Cambrian formations and sections of central Montana. The Dry Creek shale is partly emended by restricting all limestone pebble conglomerates and gray-green shales to the Pilgrim formation. This revision gives lithologic unity to both the Dry Creek and the Pilgrim in all the sections and restricts the fossil horizons to the Pilgrim. Faunal evidence substantiates the offlap character of the Dry Creek sedimentation. The relationship of the Upper Cambrian sections of central Montana to those of southern and southwestern Montana is clarified. A paleogeographic map showing the changing positions of land and sea areas during the Upper Cambrian is presented.

The early Upper Cambrian faunal zones (Cedaria, Crepicephalus, and Aphelaspis) have all been recognized in the Pilgrim. The closely spaced fossil collections from measured sections demonstrate the presence of recognizable subzone faunules in both the Crepicephalus and the Cedaria zones, and a tentative subzonal correlation of early Upper Cambrian formations in the United States is attempted. Complete faunal lists, arranged by locality and horizon, are given. Trilobites are the most abundant fossils, but inarticulate brachiopods are also common. Three species of gastropods and a calcareous alga are recorded, and fragments of crinoids and merostomes have been observed. Four genera and three new species are described from the Aphelaspis zone; 26 genera (4 new) and 42 species (32 and 2 varieties new) from the Crepicephalus zone; and 38 genera (5 new) and 70 species (43 and 1 variety new) from the Cedaria zone.