Latin retÅ«sus ‘blunt, dull, retuse (having a rounded apex with small central notch)’. According to Eggli & Newton (Etymological Dictionary of Succulent Plant Names. 2004) for the shape of the tubercles, but this is not supported by Pfeiffer’s description. Perhaps for the flattened plant body (Pfeiffer: “subglobosa”, “plattkuglige Gestalt”), which in Pfeiffer’s view distinguished the species from Mammillaria pycnacantha (= Coryphantha pycnacantha).
Maarten H.J. van der Meer (2022 Feb 05). Coryphantha retusa. Dictionary of Cactus Names. Retrieved from https://www.cactusnames.org/coryphantha-retusa