United States Geological Survey Professional Paper by U.S. Dept. of the Interior: Stratigraphy of Middle Ordovician Rocks in the Zinc-Lead District of Wisconsin, Illinois, and Iowa, 1956 (274-K). United States Geological Survey Professional Paper by U.S. Dept. of the Interior: Mississippian Ostracod of the Amsden Formation (Mississippian and Pennsylvanian) of Wyoming, 1975 (848-G).