Replacement names for two species of Agassiz, 1843 (Chondrichthyes, Xenacanthiformes), and discussion of Whitley, 1940, replacement name for Giebel, 1848 (Chondrichthyes, Xenacanthiformes), preoccupied by Müller & Troschel, 1844 (Actinopterygii, Characiformes).

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