1a. Thelocactus bicolor (Galeotti ex Pfeiffer) Britton & Rose var. bicolor
Thelocactus bicolor var. schottii (Engelmann) Krainz; T. bicolor var. tricolor (K. Schumann) F. M. Knuth
Plants few branched or un-branched, usually not deep-seated in substrate. Stems usually ovoid to cylindric at maturity, 7.5-18(-38) × 5-10(-13) cm, surface very poorly concealed to 1/2 hidden by spines; ribs 8-13, tubercles confluent into rounded ribs 15-25 mm wide; areoles spaced 8-20 mm apart along ribs. Spines red and white (older spines weather to ashy white); radial spines 9-18 per areole, longest spines 12-30 mm; adaxial (bladelike) spines 16-75(-100) × 1.5 mm; central spines (1-)4, longest spines terete to sharply angular, then abaxially keeled and adaxially flat to grooved, (13-)20-45(-60) × 0.4-1.5 mm, adaxial 3 like radials but larger. 2n = 22.
Flowering (Feb-May)Jul-Sep. Chihuahuan desert scrub, Tamaulipan thorn scrub, sedimentary, igneous, and alluvial substrates; 0-1200[-2000] m; Tex.; Mexico (Chihuahua, Coahuila, Durango, Nuevo León, San Luis Potosí, Tamaulipas, Zacatecas).
Thelocactus bicolor var. bicolor, in the strict sense, may be limited to Mexico; if so, then var. tricolor and/or var. schottii might deserve formal recognition.
