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:
Tuenger_Facetie.jpg

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