Letter from Fita to Fernández Guerra, reproduced in his Cantabria, note 8, page 126, reproduced in Historia crítica de Vizcaya y de sus Fueros, by Gregorio Balparda, according to Auñamendi Entziklopedia
Barrentsoro, Karlox Iturria (1 July 1989). "Lauburua, bizi-indarraren gurpila" [Lauburua, the wheel of life force]. zientzia.eus (in Basque). Retrieved 18 March 2022.