The main of the functions which progressively were localized in the Campo Marzio and the most teeming with consequences for the urban development of the area, was the one of the place destined to the operations of the magistrature. For this aim, the gigantic square of the Saepta, united by arcades as well as to other buildings in view of forming an enormous complex adjacent to the sacred area destined to the course of meetings and vote operations. The north-south orientation of the Saepta (due to its characteristic of area ritually "inaugurated") conditioned the successive development of the area, resulting in setting the fundamental orientation of all the Campo Marzio.
The edifice was already conceived by Julius Caesar, but started only by the triumvir Lepido and finished by Agrippa in 26 a.c. and was dedicated to the memory of the dictator and destined at receiving the citizens who were going to vote, on the occasion of the century meetings. The name derives from the ligneous divisions in which it was subdivided to facilitate the electoral operations, like the shelters for sheeps, that is why it was also called sheepfold. Because by the ascent by Octavian and after by Augustus to the tribunal authority (in 23 a.c.) the elections remained an event more or less simbolical, the edifice immediately lost the initial aim and soon became a spectacles place, surrounded by covered porches for walk and decorated with numerous works of art.
It is exactly from these works that the two longitudinal porches got their name, called "Porticus Argonautarum", the one to the west, by the Pantheon, from a cycle of pictures which magnify the enterprise of the Argonautes, and the Melagri porch, the one opposite, near the Iseo Campense, or Serapeo in which was kept the famous statue of Skopas, representing the brave killer of the wildboar of Calidone.
The large edifice measured 300 m. in length, by 120 m. in width and it covered the area inclosed by via del Gesù, via del Seminario, via dei Cestari and the Corso Vittorio Emmanuele. It was approched by means of two large doors, similar to triumphal arches, opened in the middle of the long sides. On the south there was attached the Diribitorium, where took place the ballots after the voting, and wherein were kept the archives of the same.
Restored many times, under Domitian, under Adrian, under Alessandro Severo, and finally, in the IVth century, it has now almost completetly disappeared, the few remains, still visible, belong to the age of Adrian as the Pantheon.