The Semis denomination
The semis ('half') was a small Roman bronze coin that was valued at half an as. During the Roman Republic, the semis was distinguished by an 'S' (indicating semis) or 6 dots (indicating a theoretical weight of 6 uncia).

Some of the coins featured a bust of Saturn on the obverse, and the prow of a ship on the reverse.

Initially a cast coin, like the rest of Roman Republican bronzes, it began to be struck from dies shortly before the Second Punic War 218-204 BC.

The coin was issued infrequently during the Roman Empire, and ceased to be issued by the time of Hadrian 117-138 AD.
Semis

(1) Decius AD 249-250
Obverse: IMP C M Q TRAIANVS DECIVS AVG, laureate, draped and cuirassed bust right
Reverse: Mars standing left, holding shield and spear; S C across field
Ref: RIC 128; Sear 9433
(2) Nero 65 A.D.
Obverse: NERO CLAVD CAESAR AVG GERMA, bare head right, with globe at point of bust.
Reverse: CER QVINC ROM CON around, SC in exergue, Table, seen at an angle, surmounted by an urn on the left and wreath on the right.
Ref: RIC 488
(3) Tiberius 12-14 AD
Obverse: TI CAESAR AVGVST F IMPERAT VII, Head of Tiberius facing right, wearing laurel wreath
Reverse: Altar of Lugdunum, ROM ET AVG below
Ref: RIC I (Second Edition) Augustus 246