Those broken and incomplete specimens, now preserved in the museum of Yale University, will best explain why this little book was written. The author offers it, so far as lies within him, as an authoritative and accurate account of some of the creatures of earlier ages which sought new opportunities by going down from the land into the water. So far as possible he has endeavored to make the text understandable, and, he hopes, of interest also, to the non-scientific reader. He will not apologize for such scientific terms as remain, since only by their use can precision be attained: there are no common English equivalents for them. The reader will find their explanations in the chapter on the skeleton of reptiles, and especially in the illustrations. The author has had the opportunity during recent years of critically studying nearly all the reptiles described in the following pages, but, if that were the only source of his information, the accounts of many would have been meager. He has endeavored, briefly at least, to mention the names of all those to whom we are chiefly indebted for our knowledge, but in such a work as this it is manifestly impracticable to give due credit to every one. To the friends who have been of assistance in various ways he tenders his sincere thanks: to Professor E. Fraas for photographs and the kind permission to reproduce some of his excellent illustrations; to Dr. Dreverman, of the Senckenberg Museum, for several excellent photographs for reproduction or restoration; to Dr. Hauff, of Holzmaden, for an excellent photograph of an ichthyosaur; to Dr. H. F. Osborn, of the American Museum, for permission to reproduce the spirited restoration of ichthyosaurs drawn by Mr. Knight; to Professors Schuchert and Lull, and Dr. Wieland, of Yale University; to Dr. Hay and Mr. Gilmore, of the National Museum, to Mr. Barnum Brown and Dr. McGregor, of the American Museum, and to Professor Merriam, of the University of California, for photographs and other favors. SAMUEL W. WILLISTON UNIVERSITY OF CHICAGO July, 1914 CONTENTS CHAPTER PAGE I. INTRODUCTION 1 II. CLASSIFICATION OF REPTILES 13 III. THE SKELETON OF REPTILES 19 IV. THE AGE OF REPTILES 44 V. ADAPTATION OF LAND REPTILES TO LIFE IN THE WATER 59 VI. ORDER SAUROPTERYGIA 73 Plesiosauria. Nothosauria. VII. ORDER ANOMODONTIA 102 Lystrosaurus. VIII. ORDER ICHTHYOSAURIA 107 IX. ORDER PROGANOSAURIA 126 Mesosaurus.