Dictionary of Ichthyology  

coin in the fish's mouth = a miracle by Jesus in the Gospels (Matthew 17:24-27) where the temple tax is paid by a 4-drachma coin taken from the mouth of a fish caught by   Peter on Jesus' instruction (generally assumed to be St. Peter's fish, Tilapia zillii).
Coin from mouth of fish, Augustin Tünger,
Facetiae Latinae et Germanicae
Konstanz, 1486, from Wikimedia Commons: http://commons.wikimedia.org/wiki/File:

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