Opuntia mojavensis Engelmann & J. M. Bigelow, Proceedings of the American Academy of Arts and Sciences 3: 293, 1856 
Opuntia mojavensis is an enigma. The only drawing (see below) shows 2 spine clusters and an immature fruit (see below, black and white–items 6, 7, 8), and the herbarium sheet (lectotype) is similarly depauperate. Britton and Rose report that the species is imperfectly understood; also, see the original description. The type location seems to be at or near Victorville, CA where is apparently grows on slopes above the town. See the note from The Botanical Works of George Engelmann, 1887, p 163. The plant reported here has limited similarity to O. dulcis and was observed at the summit of Mt Potosi near Las Vegas, NV.
The spines vary from 2 per areole that sweep downward to 3-4(5) per areole the stand out from the cladode at a 60 degree angle. White spines may be up to 2 inches long, whereas more upright spines (upper portion of cladode) are often yellow darkening to brown at the base. Pads are up to 6-8 inches long and 4-5 inches wide and round or oval. Cladodes turn distinctly red in freezing temperatures; cladodes dimple but do not wrinkle in freezing weather and they may remain upright over the winter. More study is needed. (Read more below thumbnails).
The source of the black and white drawing is from: “Report of Explorations for a Railway Route, Near the Thirty-fifth Parallel of North Latitude from the Mississippi River to the Pacific Ocean: by Lieutenant A. W. Whipple, assisted by Lieutenant J. C. Ives’, Part V, Number 3, Figures 6, 7, 8)” (circa 1855)