1. Grusonia aggeria (Ralston & Hilsenbeck) E. F. Anderson, Cact. Succ. J. (Los Angeles). 71: 325. 1999.
Opuntia aggeria Ralston & Hilsenbeck, Madroño 36: 226, fig. 2. 1989
Shrubs, forming clumps, 3-9 cm. Roots tuberlike, 7-8 × 2.5-3 cm. Stem segments short cylindric to clavate, 3.5-9 × 1.5-3 cm; tubercles 8-18(-22) mm; areoles circular 3-4 mm in diam.; wool yellowish white. Spines (1-)5-15 per areole, mostly in distal areoles, 3-5 cm; major 1-3 abaxial spines deflexed, usually chalky white (at least adaxially), flattened to angular-flattened, longest central abaxial spine commonly twisted or curved (at times the only spine in depauperate specimens, those mostly from Big Bend region of Texas); major 0-5 adaxial spines divergent, ascending, brown to blackish and sometimes chalky, ± terete. Glochids adaxial in areole, yellow, to 4 mm. Flowers: inner tepals bright yellow, 25 mm; filaments green; style cream; stigma lobes pale yellow-green. Fruits yellow, aging gray, cylindric to ellipsoid, 20-25(-50) × 10-15 mm, becoming dry, spineless, glochidiate; areoles 25-35. Seeds yellowish to brownish, ± circular, to 5 mm in diam., with various numbers and sizes of bumps. 2n = 22.
Flowering spring (Mar-May). Chihuahuan Desert, sandy or gravelly flats, scrub with creosote bush, lower slopes, limestone or igneous substrates; 600-1500 m; Tex.; Mexico (Coahuila).
Grusonia aggeria is based on the "type" of "Opuntia grahamii × O. schottii" described by M. S. Anthony (1956). It is not a hybrid, however, between the two tetraploid putative parental taxa; it is instead a fully fertile, diploid species.