INTRODUCTION AND ACKNOWLEDGMENTS The phacopid trilobites form a widely diverse group which first appeared in North America during the early Chazyan (Lenoir limestone). From then until the close of the Hamilton, they form an element in almost all faunas, and an occasional straggler extends their range into the Chemung. Their history in North America, therefore, is shorter than in Europe, where they first appeared in the early Ordovician and continued into the early Mississippian. Little is known of their history in other parts of the world, although members of the group are found in the Paleozoic strata of Burma, Australia, and China, and form a prominent element in the Devonian faunas of South Africa and South America. The present investigation which deals with the North American representatives of the Phacopidacea was made possible by grants from the National Research Council and the Penrose Bequest of The Geological Society of America. Its progress has been facilitated by the courteous co-operation of many persons and institutions. Among the latter, the writer is especially indebted to the United States National Museum, the Peabody Museum, Yale University, the Carnegie Museum, the Maryland Geological Survey, and the Museum of Comparative Zoology for loan of specimens for study; to the New York State Museum, the American Museum of Natural History, and Walker Museum for placing their collections at his disposal. Especial thanks are due Dr. G. Arthur Cooper of the United States National Museum, for his kindness in photographing the types in the . . .