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This book is not intended to give a full and comprehensive account of the Mesozoic mammals, and nor is it intended as a handbook for research workers studying pre-Tertiary mammals. Our intention is to give an account of the origin and evolution of certain of the characters of the Mammalia. We have tried to portray the fossils we describe as the living animals they once were, not as dead bones. Our account ends with the end of the Lower Cretaceous, since by that time the major characters of the mammals had become established. There exist a number of characters which, at the present day, are confined to the Mammalia. These include: (1) a jaw articulation formed by the squamosal and the dentary; (2) a chain of three bones, malleus, incus and stapes connecting the tympanic membrane to the inner ear; (3) the presence of hair or fur; (4) the presence of milk-glands in the female; (5) the left aortic arch is the systemic arch; (6) the phalangeal formula in both manus and pes is 2.3.3.3.3; (7) some of the teeth have more than one root. Of these characters (1) or (2) are sufficient by themselves to define a mammal; characters (6) and (7) are known to have been already in existence in some of the mammal-like reptiles - the ancestors of the mammals.