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This is an ancient Roman silver Siliqua of emperor Valens, dating to 367 - 375 A.D.
Obverse: D N VALEN S P F AVG "Our Lord Valens, Pious and Blessed August" pearl-diademed draped and cuirassed bust facing right
Reverse: VRBS-ROMA "City of Rome" Roma seated left on cuirass, holding Victory on globe and sceptre
This coin was minted at Treveri (Trier, Germany), mintmark TRPS. It had travelled across the English Channel, being found in Sussex.
DATE: 367 - 375 A.D.
MINTMARK: TRPS (Trier )
SIZE: 17.4mm dia
WEIGHT: 2.18 grams
ATTRIBUTION: RIC IX 46a.2; RSC 110b