N.H.Holmgren (Greene) 1992 - Holmgren NH (1992) A Few Varietal Combinations in Californian Keckiella and Penstemon (Scrophulariaceae). Brittonia 44(4): 481-482. doi: 10.2307/2807198.
There is no corrections for Penstemon heterodoxus var. cephalophorus yet.