The ossiculum terminale, derived from the basi-occiput and corresponding to the pro-atlas in some animals, is an anomalous ossicle at the apex of the odontoid process. In our patient, a monogoloid female, it permitted the gradual development of an anterior and rotatory dislocation of the atlas. This deformity was first recognized when the child was six years old. It subsequently progressed producing cord compression and death when the girl was eighteen years old. Autopsy findings confirmed the nature of the lesion and the fact that the transverse ligament of the atlas had displaced into the defect making the dislocation irreducible.