MARCOSIANS, or COLOBARIANS, an ancient sect in the church, forming a branch of the Valentinians. St Irenæus speaks at large of the leader of this sect, Marcus, who, it seems, was reputed a great magician. The Marcians had many apocryphal books which they held as canonical, and out of which they picked several idle fables touching the infancy of Jesus Christ. Many of these fables are still in credit amongst the Greek monks.