IX.—Two New Genera of Coccosteid Arthrodira from the Middle Old Red Sandstone of Scotland, and their Stratigraphical Distribution

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Abstract
Synopsis: Material of a coccosteid fish, from various localities in Caithness, is described asDickosteus threiplandin.g., n.sp. The fish has previously been confused withCoccosteus cuspidatusMiller ex Ag. ( =Coccosteus decipiensAg.). A diagnosis of this new form is given and comparisons made withCoccosteusshow that it is quite distinct.Coccosteus flettiWatson is also redescribed and referred to the new genusWatsonosteus. The phylogenetic relations and stratigraphical distribution of these two genera are considered, and it is shown thatDickosteusoccurs in Caithness in the Thurso Flagstone Group above the Achanarras horizon, apparently replacingCoccosteus, and has a similar range in Orkney.Watsonosteusis 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.