The term 'of a skull part' refers to two specific parts of the skull: the parietal and the inial. The parietal is an 8-letter word that refers to the bone located at the top and sides of the skull, which forms the upper part of the cranium. The inial is a 5-letter word that refers to the bone located at the back of the skull, which forms the posterior part of the cranium. Together, these two bones form the protective casing of the brain.