Hadrian AE Sestertius. 

HADRIANVS AVG COS III PP, laureate draped bust right
ADVENTVI AVG BRITANNIAE, Hadrian, standing right, holding 
roll and raising right hand, facing Britannia standing 
left sacrificing at lighted altar between them. Britannia is
depicted as a female civilian figure wearing a long robe.
No other details known. 

RIC 882; Cohen 28.

This coin is incredibly rare and less than half a dozen
are known. I found this image of the reverse on the 
internet and it was the only one with a legible legend.

Added October 2009.