In Anderson (2001, 508) and Anderson & Eggli (2011, 468), Opuntia arechavaletae Spegazzini is considered in the synonymy of Opuntia monacantha Haworth, while Hunt et al. are of a different opinion (2006, text: 197), and consider O. arechavaletae a species in its own right, although in the note accompanying the taxon, Hunt (ibidem) specifies that perhaps it is only a variant of O. monacantha. Indeed, the descriptions of the two taxa are superimposable (ibidem, 206), moreover the characteristic fruit of O. monacantha, i.e. "pear shaped" (Anderson 2001, 508; Anderson & Eggli 2011, 468), see A&M 1348, Uruguay, Cerro Largo, Laguna Merin, Reserva Privada Bagñado del Jacarè (photos 7-9), is also part of the concept of O. arechavaletae (Hunt et al. 2006, atlas: 490, 490.4). In turn, the images representing the two taxa in the lexicons, see for O. monacantha Hunt et al. (2006, atlas: 491, 491.4, 491.5) and Anderson & Eggli (2011, 468), and for O. arechavaletae Hunt et al. (2006, atlas: 490, 490.4), are referable to a single species, that of A&M 1348. For the reported evidence, we assimilate O. arechavaletae among the synonyms of O. monacantha. (Quoted from Anceschi & Magli 2021, 75)