has part has part occurs in occurs in characteristic of inheres in has modifier ossification Decreased calvarial ossification Absent ossification of calvaria Absent ossification of the calvaria (vault of the skull). Acalvaria Acalvaria is an extremely rare congenital malformation characterized by an absence of flat bones of skull, dura mater, and associated muscles in the presence of normal cranial contents and facial bones. UMLS:C4021629 Absent bone maturation of skullcap Absent ossification of skull vault HP:0005623 abnormal lacking processual parts vault of skull