D&C 27

By one definition, "A apostle is a special witness of the name of Christ who is sent to teach the principles of salvation to others.  He is one who knows of the divinity of the Savior by personal revelation and who is appointed to bear testimony to the world of what the Lord has revealed to him..."  (Mormon Doctrine)

Because the Prophet Joseph Smith received the Melchizedek priesthood before the Church was organized, I believe this is the type of apostleship the Savior is referring to as He says -

"And also with Peter, and James, and John, whom I have sent unto you, by whom I have ordained you and confirmed you to be apostles, and especial witnesses of my name, and bear the keys of your ministry and of the same things which I revealed unto them;"  (Doctrine and Covenants 27:12)