Sancho, L. G., E. Seriñá, R. Arroyo, and E. Manrique. “Chemistry and Distribution of Umbilicaria Freyi, U. Grisea Y U. Hirsuta in the Iberian Peninsula”. Acta Botanica Malacitana, vol. 16, no. 1, Dec. 1991, pp. 175-84, doi:10.24310/abm.v16i.9180.