It was an amazing melodic percussion instrument that was constructed by Hippasus of Metapontum (5rd century B.C.) to demonstrate the musical concordant intervals.
It consisted of four copper discs which were struck by a stick. The discs were of the same diametre but of different thicknesses in relation to the ratios 1, 2/3, 3/4 and 2.
Glaukus of Rhegium and Diocles were virtuous of musical themes by hitting tuned discs.

SOURCES: M.L. West, Ancient Greek Music, K. Kormiranjos, Copper-phone, the first melodic percussion instrument (article).