The curious little shield-like Crustaceans, known under the generic name of Cyclus, were first noticed by de Koninckin1841, and have been subsequently figured and described by various authors, but their exact systematic position lias never been clearly understood owing to the absence of appendages or other indications by which their affinities might be satisfactorily ascertained.

In 1868 I gave a description, with figures, of two species:' one of which, the Cyclus radialis, had been previously noticed by de Koninck and Phillips; the other, C. Bankini, was then first made known.

In 1870, under the title of “Contributions to British Fossil Crustacea,” I redescribed the above-named species, and added Cyclus bilobatns, C. torosus, C. Jonesianus, C. Wriyhtii, C. Harknetsi, C. (Halicyne) laxus, C. (Halicyne) agnotus.

The next record of the genus is to be found in the fifth part of my Monograph of British Fossil Crustacea of the order Merostomata, which records and figures the then known seven British species of Cyclus, but adds no new forms.

In 1883 Mr. B. N. Peach, F.K.S., L. and E., F.G.S., published an account of Cyclus testudo, from the Carboniferous series of Langholm, to which reference will again be made later on.

In 1893 I noticed a new British species of Cyclus, discovered by Mr. George Scott in the "Gannister seam" of the Lower Coal-measures, Old Clough Colliery, Bacup, Lancashire, which I named Cyclus Scotti.

In the same year Mr. F. E. Cowper Eeed, B.A., F.G.S., of Trinity College, Cambridge, gave a description of what he deemed to be probably a new species of Cyclus, from the Carboniferous Limestone of Settle, Yorkshire, near to, but not identical with, C. Harhiessi, which he named V. Woodwardi.