CHELYS, the common lyre of the ancient Greeks (Gr. xays, tortoise), which had a convex back of tortoise-shell or of wood shaped like the shell. According to tradition Hermes was attracted by sounds of music while walking on the banks of the Nile, and found that they proceeded from the shell of a tortoise across which were stretched tendons which the wind had set in vibration (Ho meric Hymn to Hermes, 47-51) . The word has been applied arbi trarily since classical times to various stringed instruments.