PREFACE The Pelyeosauria, or Theromorpha, of the late Paleozoic, reptiles parti

cularly characteristic of the Permo-Carboniferous redbeds of the American Southwest, form a group of major importance in the history of land vertebrates. As first recognized by Cope, they represent an early stage in the beginning of mammal-like forms. They are also the dominant types in the oldest adequately known reptilian fauna and possess many archaic features which illustrate the structure of the primitive reptilian stock. Most of the earlier work on pelycosaurs was done by Cope. He realized the significance of the material, but his brief and often inadequate de scriptions and meager illustrations failed to give the group the scientific prominence it deserved. A decade of work on the group by Case culmina ted in the publication in 1907 of his Revision of the Pelyeosauria of North America. Case figured most of the important material then known, straightened out many taxonomic snarls, and gave the first account of the general structure of a number of the more adequately represented types. Through the influence of this work, the pelycosaurs were at length generall