We now come to a consideration of the work of Joseph Leidy, one of the three great pioneers in American vertebrate paleontology, for if we disregard the work of Hitchcock and others on the fossil footprints, few of the results thus far obtained were based upon the fruits of organized research. Leidy began his publication in 1847 and continued to issue papers and books from time to time until the year 1892, having published no fewer than 219 paleontological titles, and 553 all told. His earlier paleontological researches were exclusively on the Mammalia, which were then coming in from the newly discovered fossil localities of the West. The discovery of these forms, one of the most notable events in the history of our science, will bear re-telling. The first announcement was made in 1847, when Hiram A. Prout of St. Louis published in the Journal (3, 248–250) the description of the maxillary bone of “_Palæotherium_” (= _Titanotherium proutii_)from near White River, Nebraska. This at once drew the attention of geologists and paleontologists to the Bad Lands, or Mauvaises Terres, which were to prove so highly productive of fossil forms. About the same time S. D. Culbertson of Chambersburg, Pennsylvania, submitted to the Academy of Natural Sciences at Philadelphia some fossils sent to him from Nebraska by Alexander Culbertson. These were afterward described by Leidy in the Proceedings of the Academy, together with the paleotheroid jaw, in addition to which three other collections which had been made were also placed at his disposal for study.