From the time, now nearly a century ago, when de Blainville, applying the philosophical methods he had developed for the classification of mammals to the knowledge of the structure of Echidna and Ornithorhynchus which had been obtained by Lamarck, Geoffroy St. Hilaire, and G. Cuvier, suggested that these two animals should perhaps form a group of the same order as the Marsupials and “Monodelphes,” many authors have studied their structure and discussed their affinities. Nearly all the great anatomists of the last century have at one time or another described the skull of a monotreme, either formally or in some text-book or other general work. Owing to the extreme difficulty of obtaining skulls which show sutures, their accounts vary to a very great extent. Finally, in 1901, Prof, van Bemmelen published a lengthy and magnificently illustrated account of the skulls of both monotremes which had every appearance of being a definitive description. Some years ago, when describing the skull of the “Cynodont” Diademodon, I endeavoured to institute a comparison between that animal, which in all ways makes an extremely close approach to mammalian structure, and the monotremes, admittedly the most reptilian of all mammals. To my very great surprise I found it impossible to compare the skull of Ornithorhynchus as interpreted by van Bemmelen with Diademodon. As at that time I had not sufficient material satisfactorily to undertake an independent investigation of the monotreme skull, I compared Diademodon with Dasyurus, a comparison which involves no difficulty.