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This is a nice ancient Roman silver Siliqua of emperor Constantius II struck between 18th August 353 to 6th November 355 A.D.
D N CONSTANTIVS PF AVG - pearl-diademed, draped, cuirassed bust right
VOTIS XXX MVLTIS XXXX in four lines within wreath.
This coin was minted at the Prima (1st) Officina at the Arles Mint (in France), mintmark P CON. It had traveled across the English Channel, being discovered by a metal detectorist near York.
DATE: 353 - 355 A.D.
MINTMARK: PCON (Arles, France)
SIZE: 17mm dia
WEIGHT: 2.3 grams
ATTRIBUTION: RIC VIII Arles 207.