This figure is taken from Garrucci. The cross is the old form of a hooked cross. This symbolic picture and the entire inscription was found on an ancient Christian tombstone in the catacombs of S. Callisto. The lamb resting beneath the cross represents the soul of the departed Christian. The dove presents the olive branch, the symbol of peace, as if calling to the departed one, “Rest in Peace”.
Since the 3rd century the dove is used frequently as a symbol; often it is represented as the dove of the ark, sometimes as a symbol of the faithful Christian, and again as a sign of eternal peace. Most frequently it designates the Holy Spirit.