W. J.  Dress, ?Notes on the cultivated Compositae, 6. The coneflowers: Dracopsis, Echinacea, Ratibida, and Rudbeckia?, Baileya, vol. 9, pp. 67 - 93, 1961.

W. J.  Dress, ?Notes on the cultivated Compositae, 8. Thelesperma, Bidens, Coreopsis?, Baileya, vol. 13, pp. 20 - 42, 1965.