From this origin of the instrument, the word "shell" is often used as synonymous with "lyre," and figuratively for music and poetry.  Thus Gray, in his ode on the "Progress of Poesy," says:
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        "O Sovereign of the willing Soul,
	Parent of sweet and solemn-breathing airs,
	Enchanting shell! the sullen Cares
	And frantic Passions hear thy soft control."