In cactusinhabitat.org 2010 and 2013, the A&M 315, A&M 324 and A&M 342, now identified with M. bahiensis, were attributed to Melocactus amethystinus Buining & Brederoo
May 2021

In July 2016, we made a base for about a week in the little town of Ituaçu, in the State of Bahia, Brazil, in the transitional area between the Caatinga ecoregion and the Chapada Diamantina campo rupestre. Near the town we  found some populations of Melocactus Link & Otto, see A&M 1444, Ituaçu, Rêgo Novo, 600 m, which made us reconsider our previous idea of evaluating Melocactus amethystinus Buining & Brederoo as a distinct species compared to Melocactus bahiensis Luetzelburg (Anceschi & Magli 2013a, 99). The more evident character that in Hunt et al. (2006, text: 184) distinguishes M. amethystinus (here recognized at a subspecific level), compared to M. bahiensis, are in the rib count, 9-14, triangular in section, acute for the first taxon vs. 10-13, and more or less rounded for the second. In this regard, it should be noted that the photo illustrating ssp. Bahiensis in Hunt et al. (2006, atlas: 165, 165.2), instead shows a plant with 9 ribs! According to the authors, the two taxa would also occupy distinct areas, Bahia and Minas Gerais for ssp. amethystinus, vs. Bahia and Pernambuco for ssp. bahiensis (ibidem, text: 184). Now, the population of A&M 1444 shows both individuals with more rounded ribs (photos 24-25), similar to the conception that Hunt et al. have of  ssp. bahiensis (ibidem, atlas: 165, 165.2), and individuals with ribs triangular in section, acute (photos 27-28), similar to the authors' concept of ssp. amethystinus (ibidem, atlas: 165, 165.1), as well as individuals with intermediate characters between the two (photo 21). They all carry 9 ribs, which as highlighted above is a character common to both taxa, and not a distinctive element of M. amethystinus. Furthermore, as we said at the beginning, the Ituaçu populations led us to re-evaluate the populations previously encountered in the north-east Minas Gerais and attributed to M. amethystinus (A&M 315, A&M 324, A&M 342). Recalling that the northeast of Minas Gerais should be a distinctive area of M. amethystinus only, the population of A&M 315 (Taylor & Zappi in Harley 25526; Horst 381), Brazil, Minas Gerais, Itaobim, 1 km west of town on the north side of Rio Jequitinhonha, 2009-04-19, instead shows individuals with the characters of M. bahiensis (i.e. more or less rounded ribs). In this regard, compare A&M 315 (M. amethystinus) photo 1, again with the photo that identifies M. bahiensis ssp. bahiensis in Hunt et al. (2006, atlas: 165, 165.2); the two individuals could live side by side in the same population. Since the morphological and locational distinctions between the two taxa have proved to be so fallabile, we prefer to transfer the populations previously attributed to M. amethystinus to M. bahiensis, as well as assimilating the first taxon into the synonymy of the second. (Quoted from Anceschi & Magli 2021, 73-74)