Note On A New Cyathaspidian Fish From The Upper Downtonian of Corvedale

The fragments of a Cyathaspidian fish from the Upper Downtonian of Corvedale are remarkable for their peculiar external ornamentation, which necessitates their reference to a new genus, Corvaspis. Closely arranged, smooth, and nearly uniform ridges of dentine (cosmine), as usual, cover the greater portion of shield; but a irregular network of fine grooves divides this ridge-ornament into small areas which have the false appearance of separate plates. Round the periphery, at least of the ventral shield, the smooth dentine forms a dense cluster of tubercles, which are very irregular in size and shape. The most important specimen shows the anterior two-thirds of a ventral shield, which must originally have measured about 3.5 cm. in maximum width and between 5 and 6 cm. in total length. It is broken sharply across behind, and its anterolateral margins are a little imperfect; but its anterior end is clearly hollowed by a rounded excavation. Its lateral borders are flattened, but the greater part of the shield is slightly convex from side to side. Its external ornament is especially well seen in a cellulose impression of the surface prepared by Professor L. J. Wills (PI. XIX, fig. 1). The longitudinal ridges of dentine, though subdivided into short lengths by the network of grooves, are otherwise little disturbed. In general they remain parallel and uniform in width, without any tendency to become concentric with the borders of the several areas. These areas are very irregular, without bilateral symmetry in their arrangement; and the largest are at.