Diocletian (284-305) reformed the coinage c. 293-4 by introducing some new denominations, including the "follis" (possibly known as a "nummis" in antiquity). It is a big coin of c. 27-28 mm and was surface-silvered to bring its silver content up towards 5%. The GENIO POPVLI ROMANI type is very common and remarkably inexpensive. This one came last week. Galerius as Caesar. 28-26 mm. Follis. 8.52 grams. 5:00 die-axis. Some surface-silvering remains. GAL VAL MAXIMIANVS NOB CAES, laureate head right GENIO POPVLI ROMANI, Genius standing holding patera and cornucopia. ANT• in exergue, A in right field RIC VI Antioch 59b, page 621. "c. 304-305". The portraits at Antioch are in slightly higher rounded relief than elsewhere. That spherical projection into the third dimension allows the mint of Antioch to be identified in-hand by the obverse alone, although I admit it is hard to see in this 2-D photo. If you want some impressive coins at an affordable price, consider the series of post-reform folles, especially those with the GENIO POPVLI ROMANI type. Show us some folles of Antioch, or of Galerius, or any GENIO POPVLI ROMANI coin you like.