A very nice piece.
Obverse: emperor's head in laurel wreath, to the right, in the rim legend:
M ANTONINVS PIVS AVG BRIT.
Reverse: serapis standing left, holding scepter, surrounded by legend:
P M TR P XVI COS IIII P P
Silver, diameter 19 x 20 mm, weight 3.59 g, axis 12 h.