Wormian bones, also known as intra sutural bones are extra bone pieces that occur within a suture in the cranium. These are irregular isolated bones which appear in addition to the usual centers of ossification of the cranium and, although unusual, are not rare. They occur most frequently in the course of the lambdoid suture, which is more "tortuous" than other sutures. They are also occasionally seen within the sagittal and coronal sutures. A large wormian bone at lambda is often called an Inca bone (Os Incae), due to the relatively high frequency of occurrence in Peruvian mummies. Another specific wormian bone, the pterion ossicle, sometimes exists between the sphenoidal angle of the parietal bone and the great wing of the sphenoid bone.