Partial agenesis of the olfactory apparatus or arhinencephaly is usually associated with other gross manifestations of cerebral malformation. The subject has recently been discussed by Stewart (1939), who described the rare variety in which absence of the olfactory bulbs and tracts constituted the main abnormality. Intermediate between this, the mildest form of arhinencephaly, and the cyclopian brain in which the endbrain fails to divide into hemispheres and cranial nerves other than the first are also affected, comes a transitional group in which a partial separation of the hemispheres has taken place and arhinencephaly is present in varying degree. The present case is an example of this intermediate variety, and owing to the rarity of the condition we have thought it worthy of record, though a lack of reliable clinical and post-mortem information somewhat reduces its value. The brain was found amongst a number at the Hereford County and City Mental Hospital.