"she thought of the first great beast of the Apocalypse, the winged lion with his paw upon the Gospel, Saint Mark"

From the King James Bible, Book of Revelation, Chapter 4, Verses 5-7:

And the first beast was like a lion, and the second beast like a calf, and the third beast had a face as a man, and the fourth beast was like a flying eagle.And the four beasts had each of them six wings about him; and they were full of eyes within: and they rest not day and night, saying, Holy, holy, holy, Lord God Almighty, which was, and is, and is to come.And when those beasts give glory and honour and thanks to him that sat on the throne, who liveth for ever and ever.

These four beasts, respectively, are said to represent Mark, Luke, Matthew and John, the authors of four of the Gospels.  These also tie into the Old Testament descriptions of Ezekiel and David, which speak of "winged creatures" in relation to an Apocalypse.  The tone of the four New Testament Gospels are said to be demonstrated by the creature each writer represents: Mark's Gospel begins with a description of John the Baptist crying out in the wilderness, and the strong opening verse of "The beginning of the gospel of Jesus Christ, the Son of God; As it is written in the prophets, Behold, I send my messenger before thy face, which shall prepare the way before me" has been compared to a lion's roar in its power and authority.  Matthew is a man because of the recognisably human way he describes Jesus; John is an eagle because of the ethereal qualities of his writing.  Luke, the calf, is considered the be the most forthright and 'bullish' in terms of creating an orderly document to give to his community. 

Mark is the patron saint of Venice, reportedly because in the 9th Century two merchants stole his remains from Alexandria and brought them to the city.  He is depicted in the city's logo, and a statue of him in the form of a lion is situated on the clock tower above St. Mark's Square