Three species assigned to the same nominal genus of Paleozoic xenacanthiform shark have been combined with the name Orthacanthus gracilis (Chondrichthyes, Xenacanthiformes, Orthacanthidae). Orthacanthus gracilis (Giebel, 1848), which was originally combined as Chilodus gracilis Giebel, 1848, is the senior synonym; it has priority over both Orthacanthus gracilis (Newberry, 1857), which was originally combined as Diplodus gracilis Newberry, 1857, and Orthacanthus gracilis Newberry, 1875a. Proposed species-group replacement names are Orthacanthus lintonensis nom. nov. for O. gracilis (Newberry, 1857) and Orthacanthus adamas nom. nov. for O. gracilis Newberry, 1875a. Chilodus gracilis Giebel, 1848 is designated as the type species of Chilodus Giebel, 1848; this species becomes the type species for Giebelodus Whitley, 1940, which is a replacement name for Chilodus Giebel, 1848 (preoccupied by Chilodus Müller & Troschel, 1844, Actinopterygii). Giebelodus Whitley, 1940 is a junior subjective synonym of Orthacanthus Agassiz, 1843.