The Genus Phialaspis and the “ Psammosteus Limestones”

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Abstract
Summary: 1. At least two species of the genus Phialaspis are represented among the ostracoderm remains from the Downtonian commonly identsed as “ Psommostrus anglicus ”, P. symondsi and a small new form, P. pococki , of which a distinct variety and a sub-species are also known. 2. Seventeen records of the genus have been established, which, apart from one , in Scotland, are all in the Anglo-Welsh area. The localities are recorded and the associated faunas listed. 3. It is pointed out that the two species have never been found together, although the localities are interspersed : at all those yielding P. symondsi , ten in number, the beds are in Wickham King's stage 1.8 (“ Psammosteus Limestones ”) ; whereas five of the seven occurrences of P. pococki are in earlier strata, the age of the fifth being disputed and of the sixth unknonn. The possible use of these species for zoning is noted. 4. It is considered that for the most part the Downtonian and Dittonian vertebrate faunas were not autochthonous, but were of freshwater origin. 5. The new specimens of Phialaspis are described in detail. Almost the whole of the carapace is now known, chiefly from specimens from two new localities, Gardener's Bank, Salop, and Sharpness, Gloucestershire, and this shows a number of pteraspid and, to a lesser extent, cyathaspid features ; but it also shows specialized characters that are peculiar to the genus, which is accordingly made the type of an independent group of heterostracans. 6. What is believed to be the earliest instance of vertebrate palaeopathology is recorded.

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