_The endeavor has been made to indicate, at the end of each chapter, the museums in which the best examples of the animals described may be seen, and also some book or article in which further information may be obtained. As this book is intended for the general reader, references to purely technical articles have, so far as possible, been avoided, and none in foreign languages mentioned._ _For important works of reference on the subject of paleontology, the reader may consult "A Manual of Paleontology," by Alleyne Nicholson and R. Lydekker, a work in two volumes dealing with invertebrates, vertebrates, and plants, or "A Text-Book of Paleontology," by Karl von Zittel, English edition, only the first volume of which has so far been published. An admirable book on the vertebrates is "Outlines of Vertebrate Paleontology," by Arthur Smith Woodward. It is to be understood that these are not at all "popular" in their scope, but intended for students who are already well advanced in the study of zooelogy._ ANIMALS OF THE PAST I FOSSILS, AND HOW THEY ARE FORMED "_How of a thousand snakes each one Was changed into a coil of stone._" Fossils are the remains, or even the indications, of animals and plants that have, through natural agencies, been buried in the earth and preserved for long periods of time. This may seem a rather meagre definition, but it is a difficult matter to frame one that will be at once brief, exact, and comprehensive; fossils are not necessarily the remains of extinct animals or plants, neither are they, of necessity, objects that have become petrified or turned into stone. Bones of the Great Auk and Rytina, which are quite extinct, would hardly be considered as fossils; while the bones of many species of animals, still living, would properly come in that category, having long ago been buried by natural causes and often been changed into stone. And yet it is not essential for a specimen to have had its animal matter replaced by some mineral in order that it may be classed as a fossil, for the Siberian Mammoths, found entombed in ice, are very properly spoken of as fossils, although the flesh of at least one of these animals was so fresh that it was eaten. Likewise the mammoth tusks brought to market are termed fossil-ivory, although differing but little from the tusks of modern elephants. Many fossils indeed merit their popular appellation of petrifactions, because they have been changed into stone by the slow removal of the animal or vegetable matter present and its replacement by some mineral, usually silica or some form of lime. But it is necessary to include 'indications of plants or animals' in the above definition because some of the best fossils may be merely impressions of plants or animals and no portion of the objects themselves, and yet, as we shall see, some of our most important information has been gathered from these same imprints.