They are also found in habitats. including: pentalophus, grandis,pulchellus ssp.sharpii, leucacanthus, waldeisii,(color cream) .
E. rigidissimus ssp.rigidissimus white flower, was also found in southern Arizona.
The plant in the picture is E. rigidissimus ssp.rubispinus cv. K.W. Beisel
There is also E. fitchii ssp.fitchii white flower but I do not know whether it is a cultivar.