In this book we have the chief results of Williston's half-century of exploration and research in the field of vertebrate palaeontology. Here we find the gist of his earlier researches upon the mosasaurs, plesiosaurs, and pterosaurs of the marine Cretaceous of Kansas, the substance of his later and fundamental discoveries among the primitive reptiles of the Permian of Texas, and the epitome of his last, comprehensive survey of the evolution of the Reptilia as a whole. The writing of this book was thus the culminating effort and achievement of his inspiring career.^[1] Death overtook him before the final revision and completion of this work, but happily not before he had finished the greater part of the text and had made for it with his own pen a large series of new and excellent line-drawings.

In accordance with Williston's wishes the writer undertook to put his last work in shape for the publisher and to see it through the press. For the long delay since 1918 there have been too many causes to be profitably set forth in detail. The University of Chicago Press, which had published Williston's earlier books, repeatedly found itself unable to accept this one notwithstanding its good will, and private publishers proposed conditions that were not acceptable, either to the Williston Memorial Committee, or to Professor Williston's family. After much unsuccessful correspondence in various directions, the sad plight of Williston's still unpublished work came to the notice of Professor Thomas Barbour of Harvard University, and through his good offices the Harvard University Press now has the honor of publishing the "Osteology of the Reptiles."

The new drawings that Williston made for this book have been supplemented by many other illustrations, mostly from Williston's earlier works, which were needed to illustrate the present text. The University of Chicago Press has courteously loaned many of these cuts, while others have been copied from the original publications of the authors to whom they are credited. The American Museum of ​Natural History, with the cordial approval of President Osborn, has at all times given indispensable support in the work of making ready this book for the press. Special acknowledgment is due to Mrs. E. H. Fink and Mrs. C. P. Meadowcroft of the Museum. Dr. G. K. Noble, Curator of Herpetology, has supplied critical notes on the sections dealing with recent reptiles.

Every effort has been made to keep intact the spirit and letter of the original text, and in the few places where corrections or emendations seemed necessary they have been placed in square brackets, as have also the footnotes that record some of the more conspicuous discoveries and advances since 1918.