A new ostracode species, Paraparchites mazonensis, is described from Mazon Creek concretions in the Middle Pennsylvanian Francis Creek Shale of Savage, 1927 (Desmoinesian = Westphalian C) in northeastern Illinois. The occurrence of this genus suggests a brackish water environment in the areas of formation of the ostracode-bearing ironstone concretions Includes bibliographical references (p. 51) A new ostracode species, Paraparchites mazonensis, is described from Mazon Creek concretions in the Middle Pennsylvanian Francis Creek Shale of Savage, 1927 (Desmoinesian = Westphalian C) in northeastern Illinois. The occurrence of this genus suggests a brackish water environment in the areas of formation of the ostracode-bearing ironstone concretions Fieldiana series has been published as Geological Series by Field Columbian Museum (1895-1909) and Field Museum of Natural History (1909-1943), and as Fieldiana: Geology by Chicago Natural History Museum (1945-1966) and Field Museum of Natural History (1966-)