Types of malformations Type I The cerebellar tonsils herniate inferiorly to the plane of the foramen magnum occipitalis, with conical herniation into the spinal canal; the most common. Type II: the cerebellar tonsils and the contents of the posterior cranial fossa, including the brainstem, the fourth ventricle, and the cerebellar vermis herniate inferiorly to the level of the level of the foramen magnum occipitalis; it is often combined with hydrocephalus. Type III: Cerebrospinal bulge including the cerebellum and the contents of the posterior cranial fossa. Type IV: cerebellar hypoplasia.