Totally agree! One could further split the Principate into (at least) two phases, roughly before and after 260 AD: Until the joint reign of Valerian and Gallienus people could see a continuity in the coinage familiar since centuries: - Silver coins including a relevant share of silver - The classic AE denominations Sestertius, Dupondius and As - Aurei of some sort of weight standard - Provinval coinage - no military bust types (except on Medallions) Beginning with the sole reign of Gallienus it all fell apart: - no more true silver coins, just billon Antoniniani - no more Sestertii, Dupondii and Asses (except for special occasions) - Aurei of as little as 1 gram - the end of the provincial coinage - a large variety of military busts