From the mosaic "Andromeda and Perseus".

From Enc. Britt.: Perseus was the son of Zeus and Danaë, the daughter of Acrisius of Argos. As an infant he was cast into the sea in a chest with his mother by Acrisius, to whom it had been prophesied that he would be killed by his grandson. A [..] deed attributed to Perseus was his rescue of the Ethiopian princess Andromeda when he was on his way home with Medusa's head. Andromeda's mother, Cassiopeia, had claimed to be more beautiful than the sea nymphs, or Nereid ; so Poseidon had punished Ethiopia by flooding it and plaguing it with a sea monster. An oracle informed Andromeda's father, King Cepheus, that the ills would cease if he exposed Andromeda to the monster, which he did. Perseus, passing by, saw the princess and fell in love with her. He turned the sea monster to stone by showing it Medusa's head and afterward married Andromeda.