Constantine II, Rome, AE 3. 318-319 AD. 

CONSTANTINVS IVN NOB C, Laureate, draped, cuirassed bust right.
ROMAE AETERNAE, Roma seated right on shield, inscribing X-V on shield on her lap. 
P-R across fields. 
Mintmark RQ.

RIC VII Rome 156. _rome_RIC_VII_156_T

Contributed by Scott of joviel.com.
Joviel.com is a great site written with lots of humour.