Detail of clipeus on a sarcophagus from the Castello at Ostia.
The clipeus (central roundel) on this sarcophagus has been left as a rough boss. It was presumably left like this to be carved into a portrait or other detail but for some reason never was. This might because the customer bought it in this condition and decided against having it finished or it might be that it was needed suddenly. The rest of the relief is similarly unfinished which might lend support to the second of these suggestions.