The unarmored, hadrosaurian dinosaurs of the late Cretaceous of N orth America constitute an interesting group about which much has been written and many species described. A review of this literature showed many different angles of approach, and it was in part to reconcile these various descriptions and reduce them to certain comparable common factors that this monographic study was undertaken. Aside from the mere compilation of the literature of these dinosaurs, redescriptions were prepared, nearly always in the presence of the original types and such other asso ciated material as had come to light since the species was named.The authors also undertook as complete a m orphological study of the animals as the circumstances permitted, learning what they could of the mechanics of the skeleton and teeth, the musculature and integument, the nervous system and sense organs, and the probable functions of these various parts in the living animal. They further endeavored to imagine the reconstructed creatures in their appropriate environment-physical, climatic, vegetal, and animate-and to picture them and their manner of life as animate beings of a vanished age. An account of their dis tribution both in time and space is given, as well as a discussion of their probable phylogeny and the trend of their evolution.

Skeleton of Pentaceratops fenestratus, lateral view 1 12 41. Skull of Triceratops obtusus, lateral view 126 42. Squamosal of Torosaurus latus, ventral view 131 THE CERATOPSIA University of Kansas for information concerning the Triceratops skull there, and to Mr. E. S. Riggs of the Field Museum of Natural History and Mr. F. W. Kohler of the Carnegie Museum for similar aid, as well as to the Field Museum itself for the privilege of reproducing Charles W. Knight's mural of Protoceratofs, which forms the frontispiece. Mr. J. A. Allan of the University of Alberta has kindly furnished the photograph of Chasmosaurus reproduced in Plate VII B.

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DOLLo (1884) in his fifth note on the Dinosaurians of Bernissart has discussed the muscles of mastication of certain dinosaurs in comparison with those of a rodent, a crocodile, and achameleon. This work has suggested to the writer a further study of the musculature of the skull, especially in the Ceratopsia, With a View of gaining an insight, if possible, into the origin of the peculiar defensive cranial armor of this remarkable group. These studies are based more particularly on the type skulls of Triceratops serratus and T. 12700748248 Marsh Which are preserved in the Peabody Museum at Yale, supplemented by an admirable palate, referred to the former species, in the American Museum of N atural History. The chief basis for comparative study has been the modern chameleon, Which, as the author Will show, exhibits some very remarkable points of convergence in structure toward the Ceratopsia. ' PART I. Jlluscula'ture. Jl/[uscles 0f mastication.