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This is a nice ancient Roman silver Siliqua of emperor Constantius II struck between 355-360 AD.
Obverse: D N CONSTANTIVS PF AVG "Our Lord Constantius Pious Fortunate August" - pearl-diademed, draped, cuirassed bust right
Reverse: VOTIS XXX MVLTIS XXXX "vows (prayers) on the Thirtieth anniversary (of the emperor's rule), and for his (anticipated) Fortieth anniversary" in four lines within the wreath.
This coin was minted at the Prima (1st) Officina at the Arles Mint (in France), mintmark P CON. It had travelled across the English Channel, being discovered by a metal detectorist in Cambridgeshire.
DATE: 337 - 361 AD. (this coin minted between 355 - 360 AD)
MINTMARK: PCON (Arles, France)
SIZE: 17.73mm dia
WEIGHT: 1.45 grams
ATTRIBUTION: RIC VIII Arles 207.
PROVENANCE: Private collection Peterborough.