The following research is one which has been carried out on the lines similar to that previously published by two of us in relation to the brain of the Lemur. A series of sections of the cerebral cortex has been examined in order to map out the extent and boundaries of the types of cell-lamination observed. It is now well known that these differences are correlated with differences in function, and this method of histological localisation of function (as it may be termed) has been controlled by the physiological method of stimulation. The histological portion of the work has been carried out by two of us (M. and S.), and the physiological experiments were performed at King's College, London, by the third (H.). Brief Introductory Remarks upon Correlation of Structure and Function .—The smooth brain of the Marmoset stands in structural development in some respects midway between the smooth brain of the Lemur and the convoluted brain of platyrrhine Apes higher in the zoological scale, e. g . Macacus. There are certain facts in the morphology, mode of life, and habits of these three types of animals which may be correlated with the differences in structure of their brains. The Lemur is an arboreal animal, and being nocturnal and insectivorous, it depends largely upon smell as a guiding sense. It is not surprising, therefore, to find large olfactory nerves and a relatively large area of archi-cortex. In the Lemur (half ape) the neo-cortex has not developed sufficiently to push the rhinal fissure downwards to the under surface of the temporal lobe as is found to be the case in the Ape.