Polyhymnia
Polyhymnia was the Muse of pantomimic dancing: the art of conveying emotions by gestures without speech. The Muse’s gesture of wrapping herself in a long mantle was taken to represent dance, although this rigidly posed statue seems barely moving.
The gesture with the mantle was actually a typical position for women and empresses to use when portraying themselves, so this sculpture was probably not intended to represent a dancing Muse at all.