An opportunity having been afforded me, through the kindness of Mr. A. H. Garrod, of examining the brain of a fine specimen of the Chacma Baboon (cynocephalus porcarius) from the Zoological Society's Gardens, London, I purpose, in the following pages, to record, so far as I may be able, the minute structure of the convolutions in the various parts of the cerebral hemispheres. It will be my endeavour in this inquiry to study the nerve elements of the cortex step by step, and layer by layer, and thus gradually to unfold its structure; to analyse and compare the varying appearances in different situations; and finally, collecting and arranging the facts thus elicited, to place them side by side with those derived from a study of corresponding parts in the human organ, and ascertain, if possible, the relations which exist between them.