IN teleost fishes the cerebellum is well developed and consists of two principal parts: the corpus cerebelli and the valvula cerebelli. The former is massive and usually arches backward; the latter is a pouch-like structure which projects forward under the tectum in the mesencephalic ventricular cavity. It has long been known1,2 that in one group of teleosts, the Mormyrids, the cerebellum attains amazing dimensions. In these forms the valvula consists of an intermediate part and two lateral lobes, and it is the latter which has greatly hypertrophied. These lobes have grown out of the ventricle of the midbrain, to become superficial structures which cover all other parts of the brain. Fig. 1 shows that the lateral lobes consist of an arched and folded basal layer on which are numerous small and regularly arranged ridges. The intermediate part of the valvula is composed of two lobules which are situated in front of the corpus. The corpus cerebelli itself is bent forward and is of average size (Fig. 1).