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This is an ancient Roman silver Siliqua of emperor Arcadius dating to 383 - 408 A.D.
DN ARCADI-VS PF AVG pearl-diademed draped and cuirassed bust right
VIRTVS ROMANORVM Roma seated left on cuirass, holding Victory on globe and inverted spear.
Originally this coin was minted at Rome (Italy), mintmark RMPS. It would have been included within the last official shipment of silver to Britain before Rome abandoned all interest in the province in 410 A.D.
This genre of siliqua is known for the period after the Romans left Britain, see 'The coin clipping phenomenon in sub-Roman Britain'. This coin was excavated in Yorkshire and is a classic example of this sub-Romano British (a.k.a.the romantic "Arthurian" period) coinage.
DATE: 407 - 408 A.D.
MINTMARK: RMPS (Rome, Italy)
SIZE: 11mm dia
WEIGHT: 0.76 grams
ATTRIBUTION: RIC IX 32b.3
PROVENANCE: Originally excavated near York