Sylvester, St., the first Pope of the name, said to have converted Constantine and his mother by restoring a dead ox to life which a magician for a trial of skill killed, but could not restore to life; is usually represented by an ox lying beside him, and sometimes in baptizing Constantine.
Definition taken from The Nuttall Encyclopædia, edited by the Reverend James Wood (1907)
Sylvester, St. * Symbolism