In the right field of the coin pictured below (RIC VII Nicomedia 43) there is an X over II and an episemon - "The last sign is clearly an episemon, epigraphically employed for S(emis) also." (RIC VII p. 12) Semis literally means half, and the semis coin was valued at half an as.
The quote below references these coins with the field marks which reduced them to "half of a nummus"
DEBASEMENT OF NUMMUS IN EAST BY LICINIUS, 321-324.
"Dionysius to Apion, greeting. The divine Fortune of our masters has ordained that the Italian coinage be reduced to the half of a nummus. Make haste, therefore, to spend all the Italian silver that you have in purchases, on my behalf, of goods of every description at whatever prices you find them. For this purpose I have dispatched an officialis to you. But take notice that should you intend to indulge in any malpractices I shall not allows you to do so. I pray, my brother, that you may long be in health. (Verso) I received the letter from the officials on the eight of the month Pharmouthi."
Source: Letter in Archive of official Theophanes, c. 321 (P. Rylands IV. 607). Translation from L. C. West and A. C. Johnson, Currency in Roman Byzantine Egypt (Princeton, 1944), pp. 184-185, no. 7. See M. Hendy, SBME, pp. 463-64 and R. Bagnall, Inflation in Fourth Century Egypt, pp. 12-15, who redate the papyrus from earlier dates as argued by C. H. Robert and J. G. Milne in Trans. of Inter. Num. Congress, 1936 (London, 1938), pp. 246-249 and C. H. V. Sutherland, JRS 51 (1961), 94-97.
"Two other fragmentary letters from the same archive (PSI 965 and P. Oslo III. 83) allude to the same reform. In 321 Licinius (308-324) reduced the silver content of his nummus (2.40 gs) and halved its official tariffing to 12.5 d.c. Eastern mints marked the reverses of the nummi with the value mark. The official rate of exchange was probably 1 aureus = 516 nummi sparked off a new wave of price rises until Constantine (306-337) reunited the empire and demonetized the Licinian nummus in 324. See Harl, Coinage in the Roman Economy, pp. 158-166."
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