A.  Graham and Jarzen, D. M., ?Studies in neotropical paleobotany. I. The Oligocene communities of Puerto Rico?, Annals of the Missouri Botanical Garden, vol. 56, pp. 308 - 357, 1969.

T. E.  di Fulvio, ?Recuentos cromosómicos en Heliotropium (Boraginaceae)?, Kurtziana, vol. 5, pp. 89 - 94, 1969.