chelys

thumb|Cylix of Apollo with the chelys lyre, on a 5th-century BC drinking cup ([[kylix)]]

The chelys or chelus (, , both meaning "turtle" or "tortoise") was a string instrument, the common lyre of the ancient Greeks, which had a convex back of tortoiseshell or of wood shaped like the shell. The word chelys was used in allusion to the oldest lyre of the Greeks, which was said to have been invented by Hermes. According to the Homeric Hymn to Hermes, he came across a tortoise near the threshold of his mother's home and decided to hollow out the shell to make the soundbox of an instrument with seven strings.