Constantius II. AE21. 

D N  CONSTAN-TIVS P F AVG, pearl diademed, draped, 
cuirassed bust right, Delta behind
FEL TEMP RE-PARATIO, helmeted soldier left, spearing 
falling horseman who is bearded, hair in braids, 
reaching backwards, S in left field, SMHB in ex.

RIC VIII Heraclea 88, rated common.

Dane's collection, July 2009

Additional Info:
I know of 41 different varieties of the Heraclea-struck
Fel Temp, fallen horseman coins for Constantine II.
The horseman can be bare-headed, with or without braids,
wearing a Phrygian helmet (often called a "pointed 
cap" in error), and sometimes bearded.
The bust type is always pearl diademed, draped, 
cuirassed and half of them have a delta behind.
Field marks on the reverse can be a star, Gamma,
S or M, all in the left field.
RIC References, depending on the above details:
Heraclea 61, 65, 67, 82, 83, 85, 86, 88, 90, 96
plus an unusual var. from the Boljetin hoard.

See my "Helvetica's RIC list" of all 2000+ known varieties
of this coin type, in Excel format with drop down columns
to aid identification: 
www.catbikes.ch/coinstuff/coins-ric.htm