This article deals with the identification and interpretation of the third symbol of the Miles grade on the floor mosaic of the Felicissimus mithraeum in Ostia. In previous scholarship, this symbol has usually been identified as a soldier's sling bag or, alternatively, as a Phrygian cap. The authors of this article question these identifications and hypothesize that this object might represent a bull's pelvic limb (i.e. hind-quarter) or, less likely, thoracic limb (i.e. fore-leg). They base their argument on the expert opinion of two veterinarians and also on the fact that a bull's limb is depicted on other Mithraic monuments, notably the altar of Flavius Aper at Poetovio, unlike a soldier's bag. In the second part of the article, the authors tentatively reflect on why the author of this mosaic might have chosen this particular symbol. They suggest either the possibility that this object might have played a role in Mithraic ritual(s) modelled on some episodes from a Mithras myth, or that it refers to the scene of Transitus and thus accentuates the heroic aspect of Mithras' personality in the role of deus invictus.