If you ever get a good look at the things you'll see its not even a skull, just a brain with a pair of eyeballs on a tongue. Thats it. The rest is gone. But still looks great, and they are fun as hell.
it appears to have a bone structure (you'll see the nasal cavity between the eyes as well as formed cheek bones and the top half of a jaw). also notice the eyes are attached as if they are within the structure. looks like a skull to me.