From Project Athinai
The Eponymous Heroes are the heroes who gave their name to the ten Athenians tribes (the new tribes established by Cleisthenes): Erechtheus, Aigeus, Pandion, Leos, Akamas, Oineus, Kekrops, Hippothoon, Aias (Ajax) and Antiochos. Their monument was located at the southwest corner of the Agora. It consisted of a large poros pedestal supporting a marble course projecting outward which in turn supported ten bronze statues and 2 bronze tripods (one at each end). The statues were slightly larger than life-size. The pedestal was enclosed in a fence made of poros post with poros crowning members and horizontal wooden beams. Public notices concerning each tribe, written on whitewashed wooden board, were hung on the pedestal at the feet of the appropriate hero.
The monument is mentioned by the scholiast in Aristophanes' play "Peace" which was first presented in 421 BC. Therefore, the monument probably existed during our period of reference (421 BC).
Towards 350 BC, the monument was moved in front of the Bouleuterion.
In 307/306 BC, two new tribes (Antogonis and Demetrias) were created and the monument had to be lengthened to accommodate two additional statues.
During the Roman period, part of the stone posts were replaced with marble ones.
American School of Classical Studies at Athens; The Athenian Agora: A Guide to the Excavation and Museum, 1990
Connolly, Peter and Hazel Dodge; The Ancient City - Life in Classical Athens & Rome, Oxford: Oxford University Press, 1998
Travlos, John, Pictorial dictionary of Ancient Athens, Books that matter, New York, 1971, p. 210