This species was first described by Sir P. De M. Grey Egerton as Palæoniscus monensis, in 1850, from detached scales from the Coal-measures of Anglesey. Subsequently, Dr. R. H. Traquair proved that the proper position of the fish was in his genus Rhadinichthys. Up to this date only detached scales had been known, but in the paper just mentioned Dr. Traquair described specimens showing many points in the structure of the fish which had been hitherto unknown. In my collection there are many fragmentary remains, collected from the Yorkshire Coal-measures, which show, among other interesting points, the form and ornamentation of many of the bones of the head and shoulder-girdle; while one more nearly perfect specimen shows well the form of the fish and some other points in its anatomy which have not been hitherto described, namely, the relative positions of the dorsal, anal, and ventral fins, the former being very well preserved and perfect.