X-The ostracoderm Pteraspis Kner and the relationships of the agnathous vertebrates

Ever since the attention of geologists turned to the Old Red Sandstone more than one hundred years ago, the shields and isolated plates of the Ostracoderm now known as Pteraspis have been familiar fossils to all who have collected from the lower beds of that formation in the West of England ; indeed, not only are they the commonest fossils in the Lower Old Red Sandstone, but often they are the sole identifiable organic remains present, and are locally abundant.