PRECAMBRIAN PALEOZOIC CAMBRIAN ORDOVICIAN SILURIAN DEVONIAN MISSISSIPPIAN PENNSYLVANIAN PERMIAN MESOZOIC JURASSIC CRETACEOUS CENOZOIC Thallophyta Bryophyta Tracheophyta Protozoa Porifera Coelenterata Bryozoa Brachiopada Mollusca Annelida Arthropoda Echinodermata Chordata [Illustration: Plate 12 FOSSIL PLANTS] [Illustration: THALLOPHYTES] DIATOMS × 900 ALGAE × 400 [Illustration: TRACHEOPHYTES] LEPIDODENDRON × ½ SIGILLARIA × ½ [Illustration: Plate 13 FOSSIL PLANTS TRACHEOPHYTES] NEUROPTERIS × ½ PSILOPHYTON × ⅓ CALAMITES × ½ AMELANCHIER × ½ CORDAITES × ¼ GINGKO × ½ Fairly well-preserved plant remains may also be collected from the Woodbine group of the Upper Cretaceous in north Texas, and fossil wood, most of it silicified, has been reported from rocks of almost all ages and in almost every section of the State. In addition, some of the carbonaceous clays and shales of east Texas contain large assemblages of plant leaves, which in some places are well preserved. It is also possible to find the fossilized remains of seeds, spores, and pollen. Because of their small size, these minute remains are not destroyed by the drill bit and can be brought out of deep wells without being damaged, and for this reason they are a valuable tool for the micropaleontologist. ANIMAL FOSSILS The fossilized remains of animals are very common in many of the sedimentary rocks of Texas. These remains are of many different kinds and represent the fossils of such diverse organisms as the shell of a tiny one-celled animal or the bones or tusk of a huge elephant. The fossils most commonly found, however, are the remains of invertebrate animals such as clams, snails, and corals, and it is this type of fossil that attracts the interest of most amateur collectors. It is not always easy to tell whether certain organisms are plants or animals, and because of this some scientists have suggested that these “in-betweens” be placed in a separate kingdom—the Protista. The protistans are primarily unicellular organisms and are represented by such forms as bacteria, algae, diatoms, and the protozoans (see below). But in this publication, only the plant and animal kingdoms are recognized. Phylum Protozoa This phylum is composed of simple one-celled animals many of which have no shell or external body covering. Some, however, have external hard parts that can become fossilized, and these forms are quite useful microfossils. CLASS SARCODINA.—