Pausanias' Description of Greece, Volume 2

Now the country between Elis and Sicyonia which borders on the Corinthian Gulf is called in our day Achaia from its inhabitants, but in ancient times was called Ægialus and its inhabitants Ægialians, according to the tradition of the Sicyonians from Ægialeus, who was king of what is now Sicyonia, others say from the position of the country which is mostly on the sea-shore. After the death of Hellen his sons chased their brother Xuthus out of Thessaly, accusing him of having privately helped himself to their father’s money. And he fled to Athens, and was thought worthy to marry the daughter of Erechtheus, and he had by her two sons Achæus and Ion. After the death of Erechtheus he was chosen to decide which of his sons should be king, and, because he decided in favour of Cecrops the eldest, the other sons of Erechtheus drove him out of the country: and he went to Ægialus and there lived and died. And of his sons Achæus took an army from Ægialus and Athens and returned to Thessaly, and took possession of the throne of his ancestors, and Ion, while gathering together an army against the Ægialians and their king Selinus, received messengers from Selinus offering him his only child Helice in marriage, and adopting him as his son and heir. And Ion was very well contented with this, and after the death of Selinus reigned over the Ægialians, and built Helice which he called after the name of his wife, and called the inhabitants of Ægialus Ionians after him. This was not a change of name but an addition, for they were called the Ionian Ægialians. And the old name Ægialus long prevailed as the name of the country. And so Homer in his catalogue of the forces of Agamemnon was pleased to call the country by its old name,