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,^[1][2] but it probably does represent a grain-measure, and symbolizing one's ability to learn new information by having an open mind with an empty cup. Serapis was the main idol/figurehead at the Library of Alexandria during the ancient Egyptian & Roman alliance.

The modius is worn by certain deities, including the Eleusinian deities and their Roman counterparts, the Ephesian Artemis and certain other forms of the goddess,^[3] Hecate, and Serapis.^[4] On some deities it represents fruitfulness.^[5]

It is thought to be a form mostly restricted to supernatural beings in art, and rarely worn in real life, with two probable exceptions. A tall modius is part of the complex headdress used for depictions of Egyptian royal women, often ornamented variously with symbols, vegetative motifs, and the uraeus.^[6] It was also the distinctive headdress of Palmyrene priests.^[7][8]

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  Serapis wearing the modius
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  Kushan ruler Huvishka with seated Roman Egyptian god Serapis ("Sarapo") wearing the modius.^[9]
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  Sitamun, daughter of Amenhotep III, depicted wearing a modius on her throne. From the Tomb of Yuya and Thuya, 18th Dynasty
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  Statue of Meritamen, daughter of Ramesses II, wearing a modius decorated with uraei. From the Ramesseum, 19th Dynasty.
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  Ankhesenamun depicted on a box lid wearing a modius topped by a head cone and two uraei while presenting bouquets to her husband, Tutankhamun. Tomb of Tutankhamun, 18th Dynasty.