A combination of the upside-down cross and the keys to the kingdom.
According to legend, Peter requested to die on an upside-down cross as he did not deem himself worthy to be crucified in the same way as his Lord.

The chalice and snake represents the tradition that John's enemies gave him a poisoned drink. John drank from it but he did not die. God protected him. The most distinctive symbol of John the Evangelist is that of the eagle. Legend states that the eagle was the only animal to be able to look directly into the sun.