Lily Bart is so beautiful, it's as though she came from another world – an ancient Greek world, actually, if you want to get more specific. House of Mirth is peppered with references to ancient Greek mythology, usually with Lily as some gorgeous centerpiece. Take a look:
The attitude revealed the long slope of her slender sides, which gave a kind of wild-wood grace to her outline – as though she were a captured dryad subdued to the conventions of the drawing-room; and Selden reflected that it was the same streak of sylvan freedom in her nature that lent such savour to her artificiality. (1.1.119)
That BEYOND! on her letter was like a cry for rescue. He knew that Perseus's task is not done when he has loosed Andromeda's chains, for her limbs are numb with bondage, and she cannot rise and walk, but clings to him with dragging arms as he beats back to land with his burden. (1.14.44)
Um…what? The sylvan dryad stuff makes sense, but who is Perseus? Time for a little Greek Mythology Break. Andromeda was a beautiful young Greek woman. Unfortunately, her mother wouldn't stop blabbing about how beautiful Andromeda was, and the gods got angry. To appease them, Andromeda's parents decided to sacrifice their daughter to a sea monster. Andromeda was tied to a rock, but, before the monster could take a tasty bite, the young hero Perseus showed up, saved her, took her away, and married her. Selden likens Lily to Andromeda, a woman being sacrificed for her beauty, and himself to Perseus, the rescuer man. Also, there's some great sea-imagery here, which by now you should know all about.
Speaking of sea imagery, take a look at our next Greek mythology reference, taken from Gerty's perspective after she realizes that Lawrence is in love with Lily:
The mortal maid on the shore is helpless against the siren who loves her prey: such victims are floated back dead from their adventure. (1.14.98)
The sirens were Greek monsters, half-women and half-bird, which lived on a secluded island. They had beautiful voices and used to sing to lure sailors off their course and towards certain death. Gerty imagines that Selden is one such sailor, Lily is the evil, beautiful siren, and Gerty herself is a "moral maid" waiting back home for her sailor to return. In other words, she vilifies Lily for stealing her man, and pays homage to her rival's mythical beauty while doing so.