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

Original 19th century figures of Carboniferous-age xenacanthiform shark fossils from Saxony-Anhalt, Germany, and Ohio, USA A Orthacanthus gracilis (Giebel, 1848), tooth, holotype (Geiseltalmuseum Halle, GTM 1095), two views; reproduced from Giebel (1849: pl. XXIX, fig. 2a, b), 7.7 mm long. Wettin-Schichten, Wettin/Saalegebiet, Saxony-Anhalt, Germany B Orthacanthus lintonensis nom. nov., replacement name for Diplodus gracilis Newberry, 1857a, two teeth, syntypes (repository unknown); reproduced from Newberry (1875a: pl. LVIII, figs. 3, 3a), ca 5 and 13 mm long. Upper Freeport Coal, Allegheny Group, Diamond Coal Mine, Linton, Ohio, USA C Orthacanthus adamas nom. nov., replacement name for Orthacanthus gracilis Newberry, 1875a, dorsal spine, illustration is a composite based on syntypes (Orton Geological Museum, OSU 4467A, 4467B); reproduced from Newberry (1875a, pl. LIX, fig. 7), ca 71 mm long. Upper Freeport Coal, Allegheny Group, Diamond Coal Mine, Linton, Ohio, USA D Orthacanthus carbonarius (Germar, 1844), two teeth, syntypes (repository unknown) of Chilodus tuberosus Giebel, 1848; reproduction of Giebel (1849: pl. XXIX, figs. 1, 1a, 1b as), length unknown. Wettin-Löbejun, Saxony-Anhalt, Germany E Orthacanthus carbonarius (Germar, 1844), tooth, syntype (repository unknown) of Lamna carbonaria Germar, 1844; reproduced from Germar (1844: pl. 1, fig. 1), ca 20 mm long. Presumably from Saxony-Anhalt, Germany.