Thelocactus rinconensis ssp. nidulans (Quehl) Glass, Ident. Guide Threatened Cacti of Mexico 1: TH/RIN (1997).

Basionym: Echinocactus nidulans Quehl, Monatsschr. Kakt.-Kunde 21: 119 (1911).
Holotype: ex cult. hort. Quehl, Mexico?, 12 May 1911, Anonymus s.n. (B 810006896).
Synonyms: Thelocactus nidulans Br. R., Cact. 4: 9 (1923). Thelocactus lophothele var. nidulans Kladiwa & Fittkau, in Krainz, Die Kakteen, Lfg. 61 (1975). Thelocactus rinconensis var. nidulans Glass & Foster, Cact. Succ. J. (US) 49: 245 (1977). Thelocactus rinconensis ssp. nidulans (Quehl) Doweld nom. superfl., Sukkulenty 1: 30 (1999). Thelocactus rinconensis ssp. palomaensis Pavlicek & Zatloukal, Kaktusy XXXX: 11 (2004). Thelocactus lophothele ssp. nidulans Matusz., Thelocactus 180 (2011). Thelocactus lophothele ssp. palomaensis Matusz., Thelocactus 188 (2011).

Stem single, depressed or globose, 10 cm tall, 20 cm wide. Ribs 21, distinct. Tubercles conical. Areoles without glands. Central spines 3-4, 60-80 mm long, silvery, shredded, straight, subulate. Radial spines 5-11, 5-30 mm long, silvery, shredded, straight, subulate. Flowers 40-50 mm wide, white or magenta. Seeds 2.1 x 1.6 mm, testa cells tabular or slightly convex with a verrucose surface sculpture.

Mexico, Coahuila, occurring in matorral xerofilo on limestone hills, at elevations of about 1000 to 1500 metres above sea level. Its range lies within the Central Chihuahuan subregion of the Chihuahuan Desert ecoregion.

Risk assessment
Thelocactus rinconensis ssp. nidulans is placed in the category A (threatened) of the Norma Oficial Mexicana NOM-059-SEMARNAT-2010.

This is a beautiful subspecies, mainly because of its robust spination. The presence of the radial spines and the robust, shredding central spines make it easily distinguishable from ssp. rinconensis.

Palomaensis is another form, together with ssp. freudenbergeri and icamolensis, that has magenta flowers. It has been found near La Paloma, at the northern border of the Sierra Paila. It is very close to ssp. nidulans with which shares the body shape and spination, and which also occurs in the Sierra Paila. The only consistent difference is the flower colour, not sufficient to recognise it as a separate subspecies.