In the study of a few specimens of the brains of Monotremes and Marsupials, it was discovered that in spite of the published work upon the brain of Ornithorhynchus by such savants as Koelliker (1901), Ziehen (1908), and Elliot Smith (1898-1900), knowledge of the brain stem of this interesting mammal was incomplete, as judged by present-day standards. It was felt that with the introduction of more recent viewpoints, a new study of this subject might chance to give the old facts new relationships. This monograph, as one of the rapidly increasing number of analyses of the brains of vertebrates, may help to clarify in part the inter-relationships between the brains of lower forms and those of mammals. This research, begun at the suggestion of Prof. Elliot Smith, in London, was continued at the University of Chicago, upon material which he most generously loaned the author, and was brought to a conclusion in Cambridge with the use of three series of brains in the collection of Prof. J. T. Wilson.