A diverse fauna of numerous, well-preserved polyplacophorans are present in the Mississippian (Osagean) Gilmore City Formation, Humboldt Member at Humboldt, Iowa. Specimens were collected from oolitic and skeletal packstones. New taxa include Gryphochiton demissus new species, Euleptochiton ellipticus new species Platychiton gerki new genus and species, Angulochiton humboldtensis new genus and species, and Systenochiton triangulus new genus and species.