A luscious fuck-ton of human skull references.
Two of the above images are GIFs, so wait for ‘em to load.
And I just thought of something that really ought to be mentioned; A very common mistake on drawing skulls is the eye sockets. People often make ’em smooth and solid on the inside, but it’s not accurate to have ’em completely closed off. There are holes in the eye sockets that lead to the inside of the skull, ’cause the retinas connect the eyeballs to the back of the brain (so there’s obviously an open space for the nerve to travel through). Sealed-off sockets are aesthetic, and for people who don’t want to put any more work into a tangible model of one. Just be aware of that when you’re sketching it out.
[From various sources]