The morphology, texture and some optical. properties (reflectance and fluorescence), of the Ichthyorests (fish scales) have been studied in two series of bituminous shales; one of the Westphalian of Scotland and other of the Pliensbachlan of Asturias and Cantabria. Furthermore, an artificial thermal maturation on immature samples of the Jurassic series has been carried out. The set of results compared with the optical properties of the vitrinite of the same samples permits to establish a possible thermal evolution parameter for the ichthyorests in cases of absence of vitrinite