One afternoon in June, 1876, three Princeton undergraduates were lying under the trees on the canal bank, making a languid pretence of preparing for an examination. Suddenly, one of the trio remarked: “I have been reading an old magazine article which describes a fossil-collecting expedition in the West; why can’t we get up something of the kind?” The others replied, as with one voice, “We can; let’s do it.” This seemingly idle talk was, for Osborn and myself, a momentous one, for it completely changed the careers which, as we then believed, had been mapped out for us. The random suggestion led directly to the first of the Princeton palæontological expeditions, that of 1877, which took us to the “Bad Lands” of the Bridger region in southwestern Wyoming. The fascination of discovering and exhuming with our own hands the remains of the curious creatures which once inhabited North America, but became extinct ages ago, has proved an enduring delight. It was the wish to extend something of this fascinating interest to a wider circle, that occasioned the preparation of this book. The western portion of North America has preserved a marvellous series of records of the successive assemblages of animals which once dwelt in this continent, and in southernmost South America an almost equally complete record was made of the strange animals of that region. For the last half-century, or more, many workers have coöperated to bring this long-vanished world to light and to decipher and interpret the wonderful story of mammalian development in the western hemisphere. The task of making this history intelligible, not to say interesting, to the layman, has been one of formidable difficulty, for it is recorded in the successive modifications of the bones and teeth, and without some knowledge of osteology, these records are in an unknown tongue. To meet this need, Chapter III gives a sketch of the mammalian skeleton and dentition, which the reader may use as the schoolboy uses a vocabulary to translate his Latin exercise, referring to it from time to time, as may be necessary to make clear the descriptions of the various mammalian groups. Technical terms have been avoided as far as possible, but, unfortunately, it is not practicable to dispense with them altogether. The appended glossary will, it is hoped, minimize the inconvenience.