Semis

The semis literally meaning 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.

...
Wikipedia