Abnormal posterior cranial fossa morphology An abnormality of the fossa cranii posterior (the posterior fossa), which is made up primarily of the occipital bone and which surrounds to the foramen magnum. Abnormality of the posterior cranial fossa The floor of the cranial cavity is divided into three distinct depressions called the anterior, middle, and posterior cranial fossa. The posterior cranial fossa, the most posterior and deep of the three cranial fossae, accommodates the brainstem and cerebellum. Abnormalities of the posterier cranial fossa can be demonstrated by cerebral magnetic resonance imaging or computer tomography. HP:0000932 human_phenotype Posterior fossa anomaly UMLS:C3280768 Abnormality of the posterior fossa HP:0007306 Abnormality of the skull base