Plants erect, unbranched or branched and forming mounds, deep-seated in substrate or not. Roots diffuse or short taproots. Stems unsegmented, greenish [to pale blue-gray], spheric or flat-topped to short cylindric, 3-38 × 4-20 cm, glabrous; ribs usually 7-13[-25], absent in young plants [and some Mexican taxa], crests very deeply notched above each areole, thus ribs strongly tuberculate, 15-25 mm diam.; tubercles prominent, broadly rounded, conic, pyramidal, bilaterally compressed [or long decurrent]; areoles 6-20[-52] mm apart along ribs, groove connecting areole to spine cluster absent on youngest adults and conspicuous on older plants; areolar glands often conspicuous [or absent]; cortex and pith firm, not mucilaginous. Spines [1-](8-)12-30 per areole, white, yellow, or red [black], largest spines 0.2-1.5 mm wide; radial spines [0-](8-)12-20(-25) per areole, contrasting with other spines, straight [to curved, rarely almost hooked], 9-35(-45) mm, adaxial radial spines flat and bladelike or ribbonlike; central spines 0-4(-5) per areole, usually straight, sometimes somewhat curved, terete, flattened, or angled. Flowers diurnal, from adaxial or axillary extremity of areole, near stem apex, shallowly to deeply funnelform, [2.5-]4-8(-10) × [2-]4-6.5(-10) cm; outer tepals minutely fringed; inner tepals widely spreading, magenta or pink [white, yellow, or patterned with contrasting veins or transverse white band proximally and/or broad white margins and/or orange-red to crimson proximal portions], 28-50 × 5-12 mm, margins entire or fringed to denticulate; ovary scaly, hairless, spineless; scales 5-21, broad, margins scarious, minutely fringed; stigma lobes 7-13, reddish to orange or yellowish [or whitish], 4.5 mm. Fruits dehiscent through large basal pore, green to brownish purple [to magenta], spheric to short cylindric, (5-)7-18 × 6-12(-17) mm, not juicy, drying immediately after ripening, scaly, spineless, hairless; floral remnant persistent. Seeds black or very dark brown, short cylindric to pyriform, sometimes constricted above base, 1.4-2.5 × 1-1.75 mm; testa cells convex [or flat, sometimes with shiny, raised, reticulate sculpture of slightly protruding, narrow, straight anticlinal walls]. x = 11.
Anderson, E. F. 1987. A revision of the genus Thelocactus B. & R. (Cactaceae). Bradleya 5: 49-76. Glass, C. and R. A. Foster. 1977. The genus Thelocactus in the Chihuahuan Desert. Cact. Succ. J. (Los Angeles) 49: 213-220, 244-251.
|[Greek thele, nipple, and Cactus, an old genus name, in reference to the tubercle shape]|