Marble bust of Serapis wearing a modius (Louvre)
The modius is a type of flat-topped cylindrical headdress or crown found in ancient Egyptian art and art of the Greco-Roman world. The name was given by modern scholars based on its resemblance to the jar used as a Roman unit of dry measure, but it probably does represent a grain-measure, and symbolized powers over fecundity in those wearing it.
A serapeum (Greek serapeion) was any temple or religious precinct devoted to Serapis. The cultus of Serapis was spread as a matter of deliberate policy by the Ptolemaic kings, who also built an immense serapeum in Alexandria.
the unconnected Babylonian god Ea (Enki) was titled Serapsi, meaning “king of the deep”, and it is possible this Serapis is the one referred to in the diaries. The significance of this Serapsi in the Hellenic psyche, due to its involvement in Alexander’s death, may have also contributed to the choice of Osiris-Apis as the chief Ptolemaic god.