Echinocactus, Verh. Vereins. Beförd. Gartenbaues Königl. Preuss. Staaten 3: 420. 1827 sec. Vázquez-Sánchez & al. (2013)
- =Echinofossulocactus in Gard. Mag. & Reg. Rural Domest. Improv. 17: 317. 1841 syn. sec. Hunt (2006)
- =Homalocephala, Cactaceae 3: 181. 1922 syn. sec. Hunt (2006)
- Type: Homalocephala texensis
- =Emorycactus in Succulenta (Netherlands) 75: 270. 1996 syn. sec. Hunt (2006)
- Type: Emorycactus polycephalus
- =Meyerocactus in Succulenta (Netherlands) 75: 271. 1996 syn. sec. Hunt (2006)
Echinocactus turns out to be paraphyletic in recent molecular studies (Bárcenas & al. 2011; Hernández-Hernández & al. 2011; Vázquez-Sánchez & al. 2013), with E. grusonii Hildm. resolved in a separate clade from the remaining four species, yet with only low support (Vázquez-Sánchez & al. 2013) or in a polytomy (Bárcenas & al. 2011). Vázquez-Sánchez & al. (2013) re-circumscribed Echinocactus to include only four species, excluding E. grusonii and also E. polycephalus Engelm. & J. M. Bigelow but did not suggest new generic assignment for these species. The generic name Kroenleinia was recently erected for E. grusonii, but it may be premature to accept this monotypic genus in view of the numerous unresolved or poorly supported topologies in the group.