The right to coin money being one of the regalia (sovereign prerogatives), there can be no papal coins of earlier date than that of the temporal power of the popes. Nevertheless, there are coins of Pope Zacharias (741-52), of Gregory III (Ficoroni, "Museo Kircheriano"), and possibly of Gregory II (715-741). There is no doubt that these pieces, two of which are of silver, are true coins—and not medals like those distributed as "presbyterium" at the coronation of the popes since the time of Valentine (827). Their stamp resembles the Byzantine and Merovingian coins of the seventh and eighth centuries, and their square shape is also found in Byzantine pieces. Those that bear the inscription GREII PAPE — SCI PTR (Gregorii Papæ — Sancti Petri) cannot be attributed to Pope Gregory IV (827-44), because of the peculiarity of minting. The existence of these coins, while the popes yet recognized the Byzantine domination, is explained by Hartmann (Das Königreich Italien, Vol. III), who believes that, in the eighth century, the popes received from the emperors the attributes of "Præfectus Urbis". Under the empire, coins that were struck in the provinces bore the name of some local magistrate, and those coins of Gregory and of Zacharias are simply imperial Byzantine pieces, bearing the name of the first civil magistrate of the City of Rome.