Note the name "Agenor" Greek: Ἀγήνωρ, gen.: Ἀγήνορος) is said to mean "heroic, manly."("Belus" is linked to Baal.)
We see the line of Harmonia and Cadmus, son of Agenor, which seemed at first so promising, coming to a dead halt -- the critical moment was the rape of Europa -- not only was she lost to Agenor, but so were all his sons, thanks to his ultimatum ("find her or go into exile"). Interesting to note that Epaphus, son of Io, whom we last saw taunting Phaethon into doubting his paternity, became (with Memphis) the father of Libya, who with Poseidon came to be the mother of Agenor and Belus. Agenor and Beluw thus are genealogically linked to both Zeus and Poseidon.
Belus, the brother of Agenor, married Achiroe. Their line, via Danaus and Aegyptus, culminates in Perseus, hero of Book 5.
Perseus frees Andromeda (detail) by Piero di Cosimo
Afterthought: In this chart are the eponymous forebears of Ocean, Nile, Egypt, Ionia, Libya, Phoenicia, and Memphis, and the founders of Crete, Argos, Phocis, Thebes and Mycenae.