The answer is found in Gray's Anatomy, posted online here:
http://www.bartleby.com/107/35.html
"The sphenoid articulates with twelve bones: four single, the vomer, ethmoid, frontal, and...
The sphenoid bone is one of the skull bones and is located relatively deep within the skull. It is only very slightly visible from an exterior view of the skull. Looking side on to a skull it is...