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From the point of view of phylogeny, the development of cosmine superficially on the dermal elements of certain early lower vertebrates is considered to be a specialized rather than a primitive condition. An analysis of cosmine formation in the squamation of Devonian dipnoans, based on material from the Plattenkalk of Bergisch Gladbach in the Rhineland, shows that (a) separate cosmine sheets on the individual scales, bounded-off from each other by Westoll lines, conform to the pattern of areal growth as displayed in odontode distribution on the dermal elements of various other lower vertebrates, and (b) each consecutive period of cosmine formation began in certain specific scale-areas and subsequently spread to the remainder of the squamation.