Absence of the septum pellucidum (meaning translucent wall in Latin - SP), also known as the ventricle of Sylvius. The septum pellucidum is a thin, triangular double membrane separating the frontal horns of the right and left lateral ventricles of the brain. It extends between the anterior portion of the corpus callosum, and the body of the fornix and its width varies from 1.5 to 3.0 mm. The phenotype is Human Phenotype Ontology id HP_0001331 (Absent septum pellucidum). Also known as: Absence of septum pellucidum, Absence of the septum pellucidum, Agenesis of the septum pellucidum, Missing septum pellucidum.