it and Trichina spiralis which he had met with in several human subjects in the dissecting-room, since attention had been directed to it by Mr. Hilton and Professor Owen. Leuckart afterward acknowledged that he was indebted to this communication for his success in tracing the development of trichina in the hog and man.
In September, 1847, he published his first paleontological paper, entitled "On the Fossil Horse of America," in the "Proceedings" of the Academy. The existence of remains of extinct horses on the American Continent had been regarded with incredulity, in consequence of the entire disappearance of these animals in after-ages. The paper consists of descriptions and figures of specimens contained in the Museum of the Academy of Natural Sciences of Philadelphia, some of which the author regarded as belonging to the South American form, described by Owen under the name Equus curvidens, and others as indicating a new species, for which he proposed the name Equus Americanus.
His investigations on the development of cartilage-cells, the structure of the liver, of the nettling organs in hydra, the presence of the first indication of muscular fiber in the gregarines, the discovery of the eye in the perfect condition of the cirrhopoda, together with descriptions of many new forms of entozoa and entophita, miscellaneous anatomical and zoölogical notes, and a continuous series of papers entitled "Helminth ological Contributions," enriched the pages of the "Proceedings" of the Academy during the next four or five years. His elaborate memoir on the "Anatomy of Corydalus Cornutus in its Three Stages of Existence," published in the "Memoirs of the American Academy of Arts and Sciences," and his beautifully illustrated monograph entitled "A Flora and Fauna within Living Animals," issued as part of the fifth volume of the Smithsonian contributions to knowledge, merit special mention.
These communications, laboriously prepared as many of them were, did not, however, indicate the full extent of Dr. Leidy's industry. Since the publication of his paper on the fossil horses of America, much of his time had been occupied in the study of the vertebrate fossils in the museum of the Academy, or which were brought to his notice from time to time by collectors. Long before the active exploration of the West had added so immensely to our knowledge of the extinct fauna of that region, he had determined the former existence, in a tropical climate on our western slope, of the lion, the tiger, the camel, the horse, the rhinoceros, and many other forms having no immediate existing representatives.
In 1853 the Smithsonian Institution published his memoir on the extinct species of American ox, and in the following year the elaborate "Ancient Fauna of Nebraska." Other paleontological papers were published in the "Transactions of the American Philosophical Society," and many new genera and species were announced in the "Pro-
