Today I worked on my ecorche skull for about 4 more hours and did not get close to finished.... I never knew I could spend so much time on 3 inches of clay! I think I over-emphasized the "worried" look.
As difficult as it is getting all the proportions and forms correct... then it has to be symmetrical.
Above is my current setup in my studio for the anatomy drawing homework. I open every anatomy book I have and pick the best images and prop open the pages for easy reference while I am working.