The genus Thelocactus 

Thelocactus flavus Mosco & Zanovello, Cactus & Co. 3: 20 (1999).

   Thelocactus flavus Mosco & Zanovello sp. nov. a Thelocacto garciae caule prolifero, spinis radialibus paucioribus, flore flavo differt. Holotypus: Mexico, San Luis Potosí, Huizache, Kuenzler 362, cult. A. Mosco, Nov 1998 (HG-PAD).

DESCRIPTION.
   Stem caespitose and forming small clumps up to 40 cm across, 5-12 cm diam., globose, olive green to reddish; ribs indistinct; tubercles somewhat conical, basally rhomboid, 8-17 mm long, 10-18 mm broad, 11-15 mm high; areoles 5 mm long, 4 mm wide, extending, in the adaxial part, in a short groove c. 5 mm long, with extrafloral nectaries, downy, nearly glabrous in age, whitish. Central spines 4, the upper one erect, 15-30 mm long, the two lateral ones directed upwards and outwards, 50-65 mm long, the lower one porrect, 55-70 mm long, ochre, more or less dark-coloured, becoming greyish, straight or slightly curved, annulate, slightly flattened, ridged; radial spines 3-5, 7-15 mm long, present only in the abaxial part of the areole, white to ochre, becoming greyish, straight, acicular; sometimes an additional short spine is present in the adaxial part of the areole. Flowers 2.5-4.5 cm diam., yellow; pericarpel 6 mm diam., scaly; receptacular tube scaly, scales reddish gradually developing into the outer perianth segments, these yellow with a reddish midvein; inner perianth segments yellow, oblanceolate, 19-24 mm long, 4-6 mm broad, margins entire, acute, sometimes erose apically; primary stamens inserted c. 5 mm above the receptacular base; filaments white, 7-10 mm long; anthers yellow; style pale yellow, 14 mm long, 0.9 mm diam.; stigma lobes 7-9, pale yellow. Fruits round, dehiscing by a basal pore, 6-11 mm diam., scaly, dried perianth persisting, olive green to reddish. Seeds black, 2-2.9 mm long, 1.5-2 mm diam., funicle remnants conspicuous, testa cells conical.

DISTRIBUTION.
   N central San Luis Potosí, between Huizache and La Hincada; Chihuahuan Desert type vegetation on limestone soils, at elevations from 1,200 to 1,500 m.