Autor: (K. Schumann) Britton & Rose

“Cactus with nipples”, referring to the characteristic nipple-shaped tubercles of species included in this genus.
A genus of small plants, solitary or caespitose, globose with the apex depressed, or conical to cylindrical not exceeding 40 cm high and 20 cm in diameter. Ribs often indistinct, but tubercles usually well defined, rounded or conical. Areoles situated at the apex of tubercles, tubercles sometimes elongated and grooved; nectariferous glands present or absent. Spines usually straight, variable in size and colour, central spines longer and sturdier than the radiais.
Flowers diurnal, appearing at the apex of stems, funnel-shaped, with a scaly floral tube, white, yellow, pink to magenta, rarely orange to red, pollinated by insects. Fruits greenish to reddish, scaly, basally dehiscent when ripe, from a pore where seeds escape freely, floral remains persistent. Seeds black, pear-shaped, with a sunken basal hilum, testa cells tabular, convex or conical, smooth or warty. Dispersal myrmecophilous and probably also through hydrochory (pers. obs.).
The genus Thelocactus has an extensive geographic range between the southern United States and northern Mexico, and grows on quite varied mineral substrates, alluvial soils, often among grasses, between rocks, on stony, sandy, siliceous soils, limestone (eg. T. buekii subsp. matudae) or gypsum (eg. T. rinconensis subsp. nidulans, among ferns and spikemosses), in the matorral (formation consisting of spiny thickets), also in mountains, in pine forests, as part
of a plant community consisting of Cactaceae, bromeliads and other succulents, between 50 m (T. bicolor subsp. schwarzii) and 2700 m in altitude.
Mexico (Coahuila, Durango, Hidalgo, Nuevo Leon, Queretaro, San Luis Potosi, Tamaulipas, Zacatecas), USA (New Mexico, Texas).

Currently 11 recognised species (+ 14 subspecies) following Mosco & Zanovello (2000):
– Thelocactus bicolor* (Galeotti ex Pfeiff.) Britton & Rose 1922
– Thelocactus bicolor subsp. bolaensis (Runge) Doweld 1999
– Thelocactus bicolor subsp. flavidispinus (Backeberg) N.P. Taylor 1998
– Thelocactus bicolor subsp. heterochromus* (Weber) Mosco & Zanovello 2000
– Thelocactus bicolor subsp. schwarzii (Backeb.) N.P.Taylor 1998
– Thelocactus buekii (Klein) Britton & Rose 1923
– Thelocactus buekii subsp. matudae (Sanchez-Mej. & A.Lau) Mosco & Zanovello 2000
– Thelocactus conothelos* (Regel & Klein) Backeb. & F.M.Knuth 1935
– Thelocactus conothelos subsp. argenteus (Glass & Foster) Glass 1998
– Thelocactus conothelos subsp. aurantiacus (Glass & Foster) Glass 1998
– Thelocactus conothelos subsp. flavus (Mosco & Zanovello) Mosco & Zanovello 2000 (=T. panarottoanus)
– Thelocactus conothelos subsp. garciae (Glass) Mosco & Zanovello 2000
– Thelocactus hastifer* (Werderm. & Boedeker) F.M.Knuth 1935
– Thelocactus hexaedrophorus* (Lem.) Britton & Rose 1922
– Thelocactus hexaedrophorus ssp. Iloydii (Britt. & Rose ) N.P.Taylor 1998
– Thelocactus lausseri* Riha & Busek 1986
– Thelocactus leucacanthus* (Zucc. ex Pfeiff.) Britton & Rose 1923
– Thelocactus leucacanthus subsp. schmollii (Werderm.) Mosco & Zanovello 1999
– Thelocactus macdowellii* (Rebut ex Quehl) Glass 1969
– Thelocactus multicephalus Halda & Panarotto 1998
– Thelocactus rinconensis* (Poselg.) Britton & Rose 1923
– Thelocactus rinconensis subsp. freudenbergeri (Haas) Mosco & Zanovello 1999
– Thelocactus rinconensis subsp. hintonii* Luthy 1997
– Thelocactus rinconensis subsp. nidulans (Quehl) Glass 1998
– Thelocactus tulensis* (Poselg.) Britton & Rose 1923

References: "TAXONOMY of the CACTACEAE" -  ISBN 978-84-617-3692-8 (Vol. 2)