1. Anterior / Ventral – shortening of basiocciput such that clivus is short, horizontally oriented ; and displaces the plane of foramen magnum upward in relation to the spinal column. Posterior fossa structures become crowded. Often assocd wuth platybasia
2. Paramedian – maldevelopment of occipital bones and occipital condylar hypoplasia. Clivus dorsally displaced into posterior fossa and is of normal length. Clivus invagination is compensated by excessive downward curvature of lateral squamous occipital bone.