"头盖骨底部的" can be translated as "at the base of the skull" or "the bottom part of the cranium" in English.
"At the base of the skull" emphasizes the location at the lower or foundational part of the skull.
"The bottom part of the cranium" specifically refers to the lower section of the cranium (the part of the skull that encloses the brain), with "bottom" clearly indicating the position.
Choose the expression that best fits the context of your sentence.