: Echinocactus horizonthalonius var. nicholiiSN|18723]]SN|18723]] is restricted to just a few limestone mountain ranges in Sierra del Viejo in northeastern Sonora (Mexico) and in two areas in south-central Arizona (USA), the Waterman Mountains and the Vekol Mountains in southwestern Pinal County. The total population is estimated at 10,000 individuals.
Habitat and ecology: Grows in isolated localities in the calcic rocks often on north-facing slopes where ocotillos (Fouqueria splendens), saguaros (Carnegiea giganteaSN|6374]]SN|6374]]) and other succulents occours. Their major threat is mining activity. However, this habitat is also threatened by off-road vehicles which are not restricted to established roads. Although cactus collectors do not appear to be a serious problem for Nichol's turk's head cacti at this time, seed collection by commercial cactus nurseries may impact the species.

• Echinocactus horizonthalonius subs. nicholii (L.D.Benson) Doweld
• Echinocactus horizonthalonius var. nicholii L.D.Benson
• Meyerocactus horizonthalonius var. nicholii (L.D.Benson) Doweld

• Echinocactus horizonthalonius subs. nicholii (L.D.Benson) Doweld
• Echinocactus horizonthalonius var. nicholii L.D.Benson
• Meyerocactus horizonthalonius var. nicholii (L.D.Benson) Doweld

• Echinocactus horizonthalonius Lem.
• Echinocactus horizontalis C.F.Först.
• Meyerocactus horizonthalonius (Lem.) Doweld
• Echinocactus equitans Scheidw.
• Echinocactus horizonthalonius f. centrispinus (Engelm.) Schelle
• Echinocactus horizonthalonius var. centrispinus Engelm.
• Echinocactus horizonthalonius f. curvispinus (Salm-Dyck) Schelle
• Echinocactus horizonthalonius var. curvispinus Salm-Dyck
• Echinocactus horizonthalonius var. minimus
• Echinocactus horizonthalonius var. moelleri J.N.Haage ex D.Weniger
• Echinocactus horizonthalonius f. montruosa hort.
• Echinocactus horizonthalonius f. obscurispinus (Rud.Mey.) Schelle
• Echinocactus horizonthalonius var. obscurispinus Rud.Mey.
• Echinocactus horizonthalonius var. subikii Stanik & Dráb
• Echinocactus laticostatus Engelm. & J.M.Bigelow
• Echinocactus pachycornis Muehlenpf.

• Echinocactus horizonthalonius cv. Albinous (Yellow form) hort.
• Echinocactus horizonthalonius cv. Sirotoge Suiheimaru hort.
• Echinocactus horizonthalonius cv. Short Spines

: The Nichol's turk's head cactus (Echinocactus horizonthalonius subs. nicholiiSN|18722]]SN|18722]]), is a small barrel cactus normally unbranched. The blue-green stem bears spines on vertical, spiraling ridges.
Stem: Eight- ribbed, pale grey-green to bright blue-green, reaching a maximum height of 50 cm and a diameter of 20 cm. The plant always grows as a single stem, but because seedlings often grow around its base, it may appear to have multiple stems.
Areoles: Each spine cluster (areole) contains three central and five radial spines.
Flowers. Bright pink or purplish.
Blooming season: April to mid-May often with repeated blooming throughout the heat of the summer and into September.
Fruits: Covered with white, woolly hairs.

: Major references and further lectures
1) "San Xavier Development Project, Construction and Operation: Environmental Impact Statement" 1989
2) Scott Calhoun "The Gardener's Guide to Cactus: The 100 Best Paddles, Barrels, Columns, and Globes" Timber Press, 31 January 2012
3) John R. Matthews "The Official World Wildlife Fund guide to endangered species of North America" Beacham, 1992
4) N. L. Britton, J. N. Rose “The Cactaceae. Descriptions and Illustrations of Plants of the Cactus Family.” Volume 4, The Carnegie Institution of Washington, Washington 1923
5) Curt Backeberg “Die Cactaceae: Handbuch der Kakteenkunde” Gustav Fischer Verlag, Stuttgart New York 1982–1985
6) Goettsch, B.K., Gómez-Hinostrosa, C., Heil, K., Terry, M. & Corral-Díaz, R. 2013. Echinocactus horizonthalonius. The IUCN Red List of Threatened Species. Version 2015.1. <www.iucnredlist.org>. Downloaded on 12 June 2015.
7) James Cullen, Sabina G. Knees, H. Suzanne Cubey "The European Garden Flora Flowering Plants: A Manual for the Identification of Plants Cultivated in Europe, Both Out-of-Doors and Under Glass" Cambridge University Press, 11/Aug/2011
8) Edward Anderson “The Cactus family” Timber Press, Incorporated, 2001
9) David R Hunt; Nigel P Taylor; Graham Charles; International Cactaceae Systematics Group. "The New Cactus Lexicon" dh books, 2006