AbstractØrvig, T. (Section of Palaeozoology, Swedish Museum of Natural History, S‐104 05 Stockholm, Sweden).Palaeohistological notes. 3. The interpretation of pleromin (pleromic hard tissue) in the dermal skeleton of psammosteid heterostracans.Zool. Scr. 5 (1): 35—47, 1976.—The formation and histological nature of ingrowing pleromin in the dermal skeleton of Devonian psammosteids are discussed. It is found that this pleromin is not a “secondary dentine” as previously maintained. Its initial formation took place before, and was therefore not dependent on, beginning abrasion on the external surface of the dermal plates. By properties such as hypermineralization and continuous growth it is distinguishable from dentine as normally developed in teeth and odontodes. In its development and structure the hard tissue in question shows marked resemblances to the pleromin in the tooth plates of the ptyctodontidCtenurella.Phyletically, the formation of ingrowing pleromin in the psammosteids is here interpreted as the product of fusion of successive odontode generations which came to form underneath each other instead of in the opposite direction. From this point of view, the development of compact pleromo‐aspidin in the psammosteids may parallel the emergence of complex tooth plates, arising by the fusion of teeth of consecutive generations, in fishes like ptyctodontids, holocephalans and dipnoans.— Remarks are also made here on a structural peculiarity in the aspidin