Fossils are also found and collected by persons who are interested in rocks and minerals. The varied remains of plants and animals long since petrified or replaced by mineral matter have stimulated the curiosity and become a source of enjoyment to many persons, from those who merely give a passing glance to the peculiar organic structures in the rocks to those who make a serious hobby or business of collecting and classifying the unreplaceable heritage from the ancient rocks. Fossils are interesting in part because of their variety, for they include petrified wood, shells like those of oysters, fish teeth, foot-prints, amber, dinosaur eggs, coal, imprints of fern leaves, of insects, and of fishes, and the bones of small and gigantic dinosaurs and elephants. In fact, a fossil is any evidence of life in the geologic past preserved in the rocks. Missouri rocks furnish fossils ranging in size from microscopically small fish teeth to the big skeletal remains of the mastodon, an ancestor of the elephant; but the most common ones are the structures and shells of ancient clams, corals, brachiopods, crinoids, and trilobites. [Illustration: A tooth of a mastodon, about one-half natural size. (Photo courtesy of Mr. J. R. Morrison, Louisiana, Missouri.)] [Illustration: Fossils. Upper row, coral on left, trilobite on right; center row, brachiopods; lower row, coiled cephalapod, crinoid head, and a bryozoan spiral.] The accompanying photographs illustrate a few fossils that may be found within our state, but a thorough, non-technical treatment of Missouri fossils is available in a companion volume to this booklet, “The Common Fossils of Missouri” by Prof. A. G. Unklesbay, Missouri Handbook No. 4. The study of fossils, or paleontology, is a fascinating branch of geology which extends far beyond the recognition and cataloging of the specimens. It has been found that certain particular fossils occur in rocks of the ages which produce petroleum, and the search for that valuable substance has been directed in many instances by the fossil content of the rocks. Rocks of different ages carry different fossil assemblages, and a man skilled in paleontology utilizes the fossils in dating geologic history like the page numbers in a book of human history. Further, any student of present-day animals and plants is aided in his understanding of them if he knows the fossil record of their ancestors of the long geologic past. Arrow Heads and Other Indian Artifacts