The five species of genus Euthacanthus Powrie, 1864 are reduced to two species on morphological and stratigraphical evidence. Euthacanthus macnicoli Powrie, 1864 and Euthacanthus grandis Powrie, 1870 are here synonymised in the type species E. macnicoli Powrie, 1864. In a previous article, Euthacanthus gracilis Powrie, 1870 and Euthacanthus elegans Powrie, 1870 were combined in the species E. gracilis, and the fifth species, Euthacanthus curtus Powrie, 1870, was reassigned to Uraniacanthus curtus (Powrie, 1870). In this work, we give an in-depth study of the full range of morphological and histological structure of scales over the body of E. macnicoli, as well as of fin spine structure. Our study reveals new features of E. macnicoli, including a large ornamented dorsal sclerotic bone, ornament on the branchiostegal plates, a separate series of gular rays, calcified cartilage forming the jaws, and a postbranchial protruding spinose plate rather than the flat prepectoral plate previously described.