http://agathe.gr/democracy/the_council_and_magistrates.html
The Council and the Magistrates Like selection for military service, allotment to the Council was organized according to the division by tribes; 50 members from each tribe acted as a unit in the Council ... The Council and the Magistrates Like selection for military service, allotment to the Council was organized according to the division by tribes; 50 members from each tribe acted as a unit in the Council and held the presidency (prytany) of the Council for one of its 10 prytanies, with a third of their number constantly on duty and in residence in the Tholos, the round building next to the Council House (Bouleuterion) (14, 15). 14. Civic buildings on the west side of the Agora, from the southeast. 15. ... Drinking cup from the Tholos, ca. 480–460 B.C. The ligature on the bottom (delta epsilon) stands for DE[MOSION]: “public property.” 17. ... In the Hellenistic period the Metroon complex (14, 15) replaced the Old Council House and took over both its functions.