This coin is a reduced follis. The normal size follis weighs around 6 grams and this one weighs 2.1 grams. RIC calls this coin a half follis, which, to be more accurate, would be about 3 grams. I have seen others call it a quarter follis; but this coin is a little overweight to properly fit that designation. Drost, in his book on Maxentian coinage, calls this coin a third of a follis; which is spot on as far as the weight.
Romulus
A.D. 309- 312
reduced follis
17mm 2.1gm
DIVO ROMVLO N V BIS CONS; bare head right.
AETERNAE MEMORIAE; Eagle with wings spread on domed shrine with doors ajar.
In ex. MOSTS
RIC VI Ostia 59