There were seven grades of initiation into Mithraism, which are listed by St. Jerome. Manfred Clauss states that the number of grades, seven, must be connected to the planets. A mosaic in the Mithraeum of Felicissimus, Ostia Antica depicts these grades, with symbolic emblems that are connected either to the grades or are symbols of the planets. The grades also have an inscription beside them commending each grade into the protection of the different planetary gods. In ascending order of importance, the initiatory grades were: