The succession of Lower Old Red Sandstone rocks found in the Welsh Borderland, is reinterpreted to reveal a shifting pattern of facies of far greater complexity than was hitherto realized. The sequence of vertebrate faunas is examined in conjunction with the sediments, and in this way a fuller account is given of the conditions of life of these early vertebrates, than was previously possible. The boundary between the Downtonian and Dittonian Stages is revised to coincide with the major palaeontological break in the region, and this is shown to approximate to a major lithological break, which is here accepted as the division between the local rock units known as the Downton and Ditton Series.

The known Scottish psammosteids are re-examined and new material is described. As a result, although Traquair’s species Psammosteus pustulatus is retained, his P. taylori, P. tesselatus and P. sp are now referred to Psammosteus megalapteryx (Trautschold), Psammolepis undulata (Agassiz) and Psammosteus falcatus Obruchev respectively. In addition, the new name Psammosteus markae is proposed for P. tessellatus (Preobrajenski). The pattern of the sensory canal system previously unknown in the psammosteids is described from the dorsal plate of P. megalopteryx, and is shown to approximate to that known in Cardipeltis The dorsal plate of Psammolepis undulata, with its characteristic ornament arranged in scale-like tesserae, is described for the first time, as is the ventral plate, which has no tesserae. The dorsal plate of Psammosteus pustulatus is also described. This shows no superficial division into tesserae, its chief characteristic being a reduction of the dentine layer in the direction of the advanced Aspidosteus heckeri Obruchev, in which this layer is entirely absent. A new reconstruction of Psammosteus is attempted, based on the associated carapace of P. megalopteryx The orientation of its branchial plates and the proportions of its post-orbital plate are used to demonstrate the morphological changes that have occurred in the derivation of Psammosteus from Psammolepis The development of the psammosteid carapace is traced from the primitive condition seen in Tesseraspis to that of the advanced Psammosteus Westoll (1951) and Obruehev (1958) suggested two entirely different correlations between the Upper Old Red Sandstone of the Nairn-Elgin region and the Baltic sequence, both based on the contained vertebrate faunas. As the same psammosteid species are now shown to occur in both provinces, a more exact correlation becomes possible. The Nairn Sandstones are considered to be equivalent to the Podsnetogor Stage (Psl. undulata); the Whitemire and Alves Beds to the Snetogor and Shelon Ilmøn Stages (Pst. megalopteryx) and the Scaat Craig Beds to the e Stage (Pst. falcatus)

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