Key to species


1a Flowers magenta with red throat........................................................Thelocactus bicolor
   A  Areolar glands absent...............................................................ssp. heterochromus
   AA Areolar glands present
      B  Central spines 0(-1).............................................................ssp. schwarzii
      BB Central Spines 3-4
         C  Stem clustering a little, cylindrical, spines 18-29...........................ssp. bolaensis
         CC Stem single, ovoidal, spines 11-23
            D  Stem up to 20 cm high, ribs 8, spines 11-22................................ssp. bicolor
            DD Stem up to 9 cm high, ribs 13, spines 15-23................................ssp. flavidispinus
1b Flowers white, yellow, orange, red or magenta, without red throat


2a Primary filaments inserted well above the base of the nectary,
   seeds with smooth cuticle and conical testa cells......................................Thelocactus conothelos
   A  Flowers white or magenta
      B  Stem green, tubercles angled, flowers magenta (rarely white),
         seeds without evident remnants of the funiculus
         C  Spines 24-29, central ones white and frayed...................................ssp. argenteus
         CC Spines 11-24, central ones grayish, not frayed................................ssp. conothelos
      BB Stem olive-green or reddish, tubercles rounded, flowers white or light magenta,
         seeds with conspicuous remnants of the funiculus.................................ssp. garciae
   AA Flowers yellow, rarely orange or red
      B  Stem single, tubercles angled, spines 12-27, flowers yellow, orange or red.......ssp. aurantiacus
      BB Stem clustering, tubercles rounded, spines 7-9, flowers yellow...................ssp. flavus
2b Primary filaments inserted near the base of the nectary,
   seeds with granular cuticle and flat or convex testa cells


3a Central and radial spines white........................................................Thelocactus macdowellii
3b Central spines coloured, radial spines white or coloured


4a Flowers small, 25-45 mm in diameter, white with a magenta central stripe...............Thelocactus lausseri
4b Flowers with a diameter of 30-100 mm, white, magenta or yellow


5a Areolar glands present
 6a Stem erect or decumbent, cylindrical, up to 40 cm high, ribs 13-18, spines 24-30......Thelocactus hastifer
 6b Stem erect, globose, up to 15 cm high, ribs 8, spines 9-12, flowers 35-90 mm diam.....Thelocactus leucacanthus
    A  Flowers yellow.....................................................................ssp. leucacanthus
    AA Flowers magenta....................................................................ssp. schmollii
 6c Stem erect, globose, up to 15 cm high, ribs 13, spines 8-9, flowers 10-15 mm diam.....Thelocactus tepelmemensis
5b Areolar glands absent


7a Stem sunken or globose, ribs 13-21, tubercles angled,
   hypodermis with two or three layers....................................................Thelocactus rinconensis
   A  Flowers yellow......................................................................ssp. hintonii
   AA Flowers white or magenta
      B  Central and radial spines not distinct...........................................ssp. rinconensis
      BB Central and radial spines distinct
         C  Stem glaucous, spines strong, frayed, 8-17, flowers white or magenta..........ssp. nidulans
         CC Stem green, spines neither strong nor frayed, 8-10, flowers magenta...........ssp. freudenbergeri
7b Stem sunken, globose or shortly cylindrical, ribs present or absent,
   tubercles angled or rounded, hypodermis one-layered


8a Ribs indistinct, tubercles angled, central and radial spines indistinct................Thelocactus multicephalus
8b Ribs present or indistinct, tubercles rounded, central and radial spines distinct


9a Stem sunken or globose, glaucous, tubercles rounded, central spines generally absent,
   flowers white or pale magenta..........................................................Thelocactus hexaedrophorus
9b Stem sunken to shortly cylindrical, not glaucous, tubercles conical,
   central spines always present, flowers white or magenta.


10a Stem from globose to shortly cylindrical, olive-green, ribs present,
    flowers white or light pink, seeds with flat testa cells..............................Thelocactus tulensis
10b Stem sunken or globose, olive-green or reddish, ribs generally indistinct,
    flowers magenta, seeds with convex testa cells........................................Thelocactus buekii