I can't believe I never noticed this before.

Matthew 10:2-4(NKJV)

Now the names of the twelve apostles are these: first, Simon, who is called Peter, and Andrew his brother; James the son of Zebedee, and John his brother; 

Philip and Bartholomew; Thomas and Matthew the tax collector; James the son of Alphaeus, and Lebbaeus, whose surname was Thaddaeus; 

Simon the Canaanite, and Judas Iscariot, who also betrayed Him.

I don't know why but I had always assumed that the four Gospels were each written by an original apostle.  As I look at the names above I realize that only two of the four books of the Gospel are written by original apostles.  Mark and Luke, while apostles, were not part of the original 12.  Just weird that I never noticed that before.