Some isolated observations have been made from time to time upon the temperatures of the Monotremes. More recently Sutherland has published an important series of observations upon the temperatures of Echidna and several Marsupials, and upon the variation in their body-temperature with changes in that of the air. These observations of Sutherland have been referred to in some detail by Vernon, in whose paper, and in Pembrey’s chapter “On Animal-heat” in Schäfer’s ‘Text-hook of Physiology,’ and in Pichet’s article “Chaleur” in the ‘Dictionnaire de Physiologie,’ the literature of the subject is so fully collected that it will merely he necessary for me to refer to those papers directly bearing upon the present work, when discussing the experimental results. Without doubt, as pointed out by Sutherland, Monotremes and Marsupials present a stage of physiological development intermediate between the fairly accurate homoeothermism of the higher mammals, and the rudimentary indications in this direction which, according to Vernon, occur in lower vertebrates, but the conclusion of both Sutherland and Vernon that Ornithorhynchus “is nearly a cold-blooded animal,” which conclusion is based upon some observations of Macleay’s, is very far from being sustained by my experiments.