The genus

Thelocactus Britton & Rose, Bull. Torrey Bot. Club 49: 251 (Aug. 1922).
Basionym: Echinocactus subg. Thelocactus K. Schum., Gesamtbeschr. Kakt.: 429. 15 Apr 1898.
Type: Echinocactus hexaedrophorus Lemaire.
Synonyms: Thelomastus Frič, in Kreuzinger, Verzeichnis 10 (1935), nom. illeg. Torreycactus Doweld, Sukkulenty 1: 19 (1998).

Stem single or clustering, depressed, globose, ovoidal or cylindrical, 3-40 cm tall, 2-20 cm wide. Ribs distinct or indistinct. Tubercles present, rounded to conical. Areoles at the apices of tubercles, sometimes elongated into a short groove, with or without glands. Spines usually straight, variable in diameter, length and colour; radial and central spines generally distinguishable. Flowers apical, funnel-shaped, with a scaled ovary and tube, white, yellow, magenta or rarely orange to red. Fruits greenish to reddish, with the perianth remnants persistent, scaly, dehiscing at maturity by a basal pore. Seeds pyriform, with a basal hilum, black; testa cells tabular, convex or conical, outer periclinal wall with a verrucose microrelief or smooth.

Thelocactus hexaedrophorus type

The neotype of Echinocactus hexaedrophorus Lemaire
From: C. Lemaire, Iconographie descriptive des Cactees, Plate 2 (Dec. 1841).
Image courtesy Missouri Botanical Garden. Botanicus Digital Library