Bárcenas R. T., Hawkins J. A., Chiang F., Typification of four North American club-chollas: The names of four Engelmannian species from Mexico and Southwestern United States, Haseltonia, 2012, 17:66-69, doi: 10.2985/1070-0048-17.1.8
Typification of the names Opuntia bulbispina, O. clavata, O. emoryi and O. grahamii, now belonging to Grusonia. The study compares surviving herbarium material, Engelmann's descriptions and historical illustrations, and explains the choice of nomenclatural types.