The present review is divided into two parts, the first part dealing with the interpretation of the third symbol of the Miles grade on the floor mosaic of the Felicissimus Mithraeum in Ostia, the second with its contextualization within the framework of Roman Mithraism. The interpretation of the third symbol of the Miles grade as a bull's limb offered by Aleš Chalupa and Tomáš Glomb is highly convincing. The contextualization of this new discovery within the framework of Roman Mithraism, however, poses some problems, which are briefly discussed here.