Hansen P., Lophophora fricii Habermann und die umstrittene Varietät Lophophora williamsii var. decipiens Croizat, Kakteen und andere Sukkulenten, 2000, 51(9):225-232
A comparison of variation in Lophophora fricii and collection specimens identified as L. williamsii var. decipiens. The author documents stems, flowers, branching and seed coat sculpture under an electron microscope. He discusses the proposed assignment of some of the studied material to L. fricii and unclear cases. The comparison includes plant photographs, seed micrographs and the history of use of the varietal name in collections.