Anatomy Study: the Skull

This is my first full-scale drawing for my effort toward understanding human structure and anatomy. I got started from a basic shading tutorial (for an apple) at CGSociety, which recommended the skull as the next subject to try, and provided the reference.

At some point I will probably move this to scraps, but I'm hoping to get some feedback first.

14-18 hours (lost count)
Photoshop CS2, 6x8 Wacom Intuos 3
Reference source thread: [link]