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Constantine of Cornwall
http://www.catholic-forum.com/saints/saintc2i.htm
Profile from Catholic Forum gives basic information on the saint.
Constantine of Scotland
http://www.saintpatrickdc.org/ss/0311.htm#cons
Short hagiography of the king and martyr.
Martyr Constantine of Cornwall
http://www.orthodox.net/western-saints/constantine-martyr-of-cornwall.html
Sixth century. Includes traditions surrounding his life.