Thanks are due also to Dr. G. A. Cooper, United States National Museum, who prepared Plate 3 especially for this publication, and to R. T. Bird and the American Museum of Natural History for photographs used in Plates 4 and 43. Plates 38 and 39 were provided through the courtesy of Dr. J. W. Dixon, Jr., and the Geology Department of Baylor University. The other photographs were prepared by the writer. To Sarah Louise Wilson, Lamar State College of Technology, the writer gratefully acknowledges her tireless and painstaking efforts in preparing the many fine drawings which make up the balance of the illustrations. WHAT ARE FOSSILS? _Fossils are the remains or evidence of ancient plants or animals that have been preserved in the rocks of the earth’s crust._ Most fossils represent the preservable hard parts of some prehistoric organism that once lived in the area in which the remains were collected. The word fossil is derived from the Latin word _fossilis_, meaning “dug up,” and for many years any unusual object dug out of the ground was considered to be a “fossil.” For this reason some of the earlier books dealing with fossils include discussions of rocks, minerals, and other inorganic objects. There is much evidence to indicate that man has been interested in fossils since the very earliest times, and fossil shells, bones, and teeth have been found associated with the remains of primitive and prehistoric men. It is quite possible that the owners of these objects believed that they possessed supernatural powers, such as healing properties or the ability to remove curses. During the earliest periods of recorded history, certain Greek scholars found the remains of fish and sea shells in desert and mountainous regions. These men were greatly puzzled by the occurrence of these objects at such great distances from the sea, and some of them devoted considerable time to an explanation of their presence. In 450 B.C., Herodotus noticed fossils in the Egyptian desert and correctly concluded that the Mediterranean Sea had once been in that area. Aristotle in 400 B.C. stated that fossils were organic in origin but that they were embedded in the rocks as a result of mysterious plastic forces at work within the earth. One of his students, Theophrastus (about 350 B.C.), also believed that fossils represented some form of life but thought that they had developed from seeds or eggs that had been planted in the rocks. Strabo (about 63 B.C. to A.D. 20) was another important Greek scholar who attempted to explain the presence of fossils. He noted the occurrence of marine fossils well above sea level and correctly inferred that the rocks containing them had been subjected to considerable elevation.