Fl. forming a double triangle, 90mm wide; perianth segments 35mm long, ovate, blunt, yellow, with slight, pale mucro, spreading, plane, smooth and of membranaceous substance, overlapping half; the inner segments narrower, somewhat shouldered at base, with midrib showing; corona 33mm long, narrow funnel shaped, angled, mouth straight, broadly ribbed, wavy to lightly frilled, with rim crenate. 

Named after the old Spanish mission south of Tucson, AZ, called San Xavier del Bac.