Polyhymnia

Polyhymnia is a Greek goddess, daughter of the gods, Zeus and Mnemosyne.
Was in Greek mythology the Muse of sacred poetry, sacred hymn as well as pantomime. She is depicted as very serious, pensive and meditative, and often holding a finger to her mouth, dressed in a long cloak and veil and resting her elbow on a pillar. Polyhymnia is also sometimes accredited as being the Muse of geometry and meditation.
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Polyhymnia does not appear in the movie or the animated series, because it only used to Calliope, Thalia, Terpsichore, Melpomene and Clio ...
forgetting about to Urania, Euterpe and Erato and Polyhymnia.