12a. Opuntia engelmannii Salm-Dyck ex Engelmann var. engelmannii
Opuntia dillei Griffiths; O. discata Griffiths; O. phaeacantha Engelmann var. discata (Griffiths) L. D. Benson & Walkington
Stem segments circular to obovate to rhombic, 20-40 × 17-30 cm, to 1.5 times longer than wide. Spines (0-)1-5(-12) per areole, absent or at most areoles, chalky white, yellow when wetted, aging gray to black, usually with red-brown extreme bases, longest 20-50(-75) mm. 2n = 66 .
Flowering spring (Apr-Jul). Deserts, grasslands, woodlands, plains, sandy soils to rocky hillsides, lower to midslopes of mountains; 300-2700 m; Ariz., Calif., Nev., N.Mex., Tex., Utah; Mexico.
In Arizona, California, and New Mexico, Opuntia engelmannii var. engelmannii hybridizes with O. phaeacantha yielding numerous named and unnamed hexaploid forms, including O. wootonii Griffiths; the various intermediates have 2n = 66. Hybrid swarms occur in both Zion and Grand Canyon national parks; these plants were perhaps brought in to the parks by native peoples as a food source and they are associated with agaves also believed to have been imported.