First of all, looks awesome, got a cool biology notebook vibe! I would say that the proportions are a bit off here and there. For instance, the left side of the skull is more narrow than the right side, the nose holes (don't know how you'd call those) are too big, also over all the skull is a bit too long I think. Maybe next time before diving into details add some more construction lines, like a center line etc. And check: where sits the nose bottom of the nose in correlation with the height of the cheekbones, how do the corners of the eyes line up with the width of the jaw etc.