Tainoceratid and liroceratid nautilids from the Upper Mississippian Imo Formation of Arkansas, Midcontinent North America

As the third installment of our study to describe the cephalopod fauna of the Upper Mississippian Imo Formation, four species of late Chesterian (= Serpukhovian; late early Carboniferous) nautilids are recorded from the dark gray shale of the formation in northwest Arkansas, Midcontinent North America. They include a tainoceratid, Tylonautilus gratiosus (Girty), and three liroceratids, Bistrialites bicostatus (Gordon), Condraoceras? sp. and Peripetoceras kummeli sp. nov. Diagnostic features of P. kummeli separating it from comparable species are its depressed reniform whorl sections with relatively high width/height ratios, large convergent angle of the flanks, and subcentral to subdorsal siphuncular position.