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Catholic Encyclopedia
http://www.newadvent.org/cathen/09674a.htm
Pope St. Mark, reigned for less than 9 months, d. 336.
Popes Through the Ages
http://www.cfpeople.org/Books/Pope/POPEp34.htm
Profile of St. Mark.
The Ecole Glossary
http://www2.evansville.edu/ecoleweb/glossary/pmark.html
Brief entry on Pope Mark, by Karen Rae Keck.