Cope’s early work was among the amphibia and reptiles, his first paleontological paper, the description of _Amphibamus grandiceps_, appearing in 1865. This year he also began his studies of the mammals, especially the Cetacea, both living and extinct, from the Atlantic seaboard. The next year saw the beginning of his work on the material from the Cretaceous marls of New Jersey, describing therefrom one of the first carnivorous dinosaurs, _Lælaps_, to be discovered in America. In 1868 Cope began to describe the vertebrates from the Kansas chalk and three years later made his first exploration of these beds. This led to his connection with the United States Geological Survey of the Territories under Hayden, and to continued exploration of Wyoming and Colorado in 1872 and 1873. The material thus gained, consisting of fishes, mosasaurs, dinosaurs, and other reptiles, was described in the Transactions of the American Philosophical Society as well as in the Survey Bulletins. In 1875 these results were summarized in a large quarto volume entitled “Vertebrata of the Cretaceous formations of the West.” Subsequent summers were spent in further exploration of the Bridger, Washakie, and Wasatch formations of Wyoming, the Puerco and Torrejon of New Mexico, and the Judith River of Montana. The material gathered in New Mexico proved particularly valuable, and led to the publication in 1877 of another notable volume entitled “Report upon the Extinct Vertebrata obtained in New Mexico by Parties of the Expedition of 1874.” Material was now accumulating so fast as to necessitate the concentration of Cope’s own time on research, so that, while he continued to make brief journeys to the West, the real work of exploration was delegated to Charles H. Sternberg and J. L. Wortman, both of whom became subsequently very well known, the former as a collector whose active service has not yet ceased, the latter as an explorer and later an investigator of extremely high promise. As early as 1865, Cope began no fewer than five separate lines of research which he pursued concurrently for the remainder of his career. On the fishes, he became a high authority in the larger classification, owing to his researches into their phylogeny, for which a knowledge of extinct forms is imperative. On amphibia, he wrote more voluminously than any other naturalist, discussing not only the morphology but the paleontology and taxonomy as well. In this connection must be mentioned not only Cope’s exploration and collections in the Permian of Ohio and Illinois, but especially the remains from the Texas Permian, first received in 1877, upon which some of his most brilliant results were based; these of course included reptilian as well as amphibian material. His third line of research, the Reptilia, is in part included in the foregoing, but also embraced the reptiles of the Bridger and other Tertiary deposits, those of the Kansas Cretaceous, and the Cretaceous dinosaurs.