In 1888, Traquair raised the genus Thursius and placed two species in it, namely Thursius macrolepidotus, which was originally described by Sedgwick & Murchison in 1829 as Dipterus macrolepidotus, and Thursius pholidotus gen. et sp. nov. In 1948 Jarvik raised the new species Thursius moythomasi, which he described as closely related to T. macrolepidotus. All these species come from the Middle Devonian deposits of the Orcadian Basin, northern Scotland. Many more specimens of Thursius are now available for study than in Traquair’s or Jarvik’s time and it can now be shown that T. macrolepidotus and T. moythomasi represent the two end ranges of variation in one very variable species. T. moythomasi must therefore, be considered a synonym of T. macrolepidotus. It is also considered doubtful that T. pholidotus is similar enough to T. macrolepidotus to warrant its retention in the genus Thursius, but more research is required to confirm this. With the taxonomic status of Thursius better understood, coupled with intensive fieldwork and the discovery of important new, stratigraphically useful species, a revision of the stratigraphy of the Orcadian Basin is presently underway.