Growing on a piece of stone near the sea. According to the site of AFL (Association Française de Lichénologie) there are two very similar species of Diplotomma - D. hedinii and D. venustum - which differ by the K reaction on medulla, K- in the former and K+ yellow to red by small plaques (dispersed) in the latter. D. hedinii is the current name in Species Fungorum for D. hedinianum , as it appears in AFL. I performed the K test on the medulla of my specimen with a negative reaction (see the photo attached). For the spores I got the following average dimensions: Me = 16 x 7.2 um ; Qe = 2.2 (N=26).