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It is in the form of a herma, a rectangular block with the head of the god on top. In ancient Greece, hermae usually represented the god Hermes, and had the god's genitals carved on it low down where passers-by could stroke them for luck. Whether the luck was with the passer-by or the god is not recorded.
The Priapus herma, and the associated two large bronze roundels with the heads of Thyrsis and Corydon, illustrate a section of Virgil's Eclogue VII.
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Stoddart's drawing is from his website. My picture was taken at totally the wrong time of day. Next time I pass in the morning I will try and get a better photo.