occipital bone

:  a compound bone that forms the posterior part of the skull and surrounds the foramen magnum, bears the condyles for articulation with the atlas, is composed of four united elements, is much curved and roughly trapezoidal in outline, ends in front of the foramen magnum in the basilar process, and bears on its external surface behind the foramen magnum the two curved transverse superior and inferior nuchal lines as well as the median external occipital crest and external occipital protuberance