Awesome drawing there. I think what is really exceptional is the soft shading on the face. The eyes are great, like most of the eyes you draw. I really like where the skin is peeling up, nicely done.
I'm not sure if the head is centered on the neck, it looks like maybe the skull's jaw sticks out a bit off the neck. The skull is smoothly shaded, but I think it would have been neat to try and capture the texture of an actual skull. The hair loses it a little for me. I think if you blended out the ends of the hair, that would help. But I do think the hair on the left side and where it connects into the head looks good.
I hope my critique helps, but awesome job either way.
