Material of a coccosteid fish, from various localities in Caithness, is described as Dickosteus threiplandi n.g., n.sp. The fish has previously been confused with Coccosteus cuspidatus Miller ex Ag. ( = Coccosteus decipiens Ag.). A diagnosis of this new form is given and comparisons made with Coccosteus show that it is quite distinct. Coccosteus fletti Watson is also redescribed and referred to the new genus Watsonosteus. The phylogenetic relations and stratigraphical distribution of these two genera are considered, and it is shown that Dickosteus occurs in Caithness in the Thurso Flagstone Group above the Achanarras horizon, apparently replacing Coccosteus, and has a similar range in Orkney. Watsonosteus is characteristic of the Eday Flags. The stratigraphy of the Thurso Flagstone Group is critically reviewed and compared with the Orkney succession; the Ackergill Beds are shown to be the probable lateral equivalents of the Mey Beds, and the consequences of this are examined.