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) Marcus Antonius 38-37 BC
AE Semis Syria
Obverse: M•ANT•TER•COS•DES• ITER•ET•TER•III•VIR•R•P•C, bare head right.
Reverse: L•BIBVLVS•M•F•PR•DESIG, quinquereme right, with three oars and stern ending in boar's head left.
Ref: RPC I 4092
(2) Parion c. 45 BC
Obverse: female head right wearing stephane; C_G / I_P
Reverse: praefericulum; D_D
Ref: RPC I 2259; SNG France 1416
(3) Rome 211-208 BC
Obverse: laureate head of Saturn right; S
Reverse: prow of galley with corvus right; grain ear above; S / K / ROMA
Ref: Crawford 069/03a; Sydenham 310b; Ty...
(4) Rome | Anonymous 211-206 BC
Obverse: laureate head of Saturn right S
Reverse: prow of galley right S ROMA
Ref: Crawford 056/03, McCabe G1