The ultimate question here is are the skull and cranium really the same thing? I had a question over on the MP list. Are there differences in human/mouse terminology?
FMA and MA both consider skull synonymous with cranium; MA has cranium with synonym of skull and FMA has skull with synonym of cranium.
There is much disagreement in sources. Some define it as strictly the bones encasing the brain (excluding maxila/mandible and other facial bones), some define it as all bones except mandible and some define it as all of the bones of the head.
This is where I've looked thus far:
Wikipedia claims that cranium and the mandible are subparts of the skull, but does not cite any references.
Stedman's medical dictionary defines cranium as:
The bones of the head, collectively. In a more limited sense, the neurocranium, the bony brain containing the brain, excluding the bones of the face (viscerocrainum). SYN skull
cranium (kra´ne-әm) pl. cra´nia the large round superior part of the skull, enclosing the brain and made up of the cranial bones.
cranium n., pl., -ni·ums, or -ni·a (-nē-ə).
1. The skull of a vertebrate.
2. The portion of the skull enclosing the brain; the braincase.
http://www.face-and-emotion.com/dataface/physiognomy/cranium.jsp excludes all facial bones in the definition of cranium, i.e. neurocranium = cranium.