The brain of Ornithorhynchus is completely devoid of convolution, simple in structure, and similar in many respects to the brains of the embryos of higher Mammals. It is, however, strictly Mammalian and not Avian in type. It differs from most, if not from all, other Mammalian brains in many respects, of which the following are the most notable:— It is destitute of corpus callosum; the structure which has been described under this name is limited exclusively to the hippocampus, and belongs therefore to the fornix. It forms a decussation or commissure above the anterior commissure. Fibres reach this decussation from all parts of the hippocampus, both in front and behind. The fibres from the decussation turn downwards in three sets (I) in front of the anterior commissure; (2) between the anterior commissure and the soft commissure; (3) above the soft commissure.