The arthropods are one of the more advanced groups of invertebrates, and they are known from the Cambrian to the Recent (Pl. 34). Modern representatives of this group include the crabs, shrimp, crayfish, insects, and spiders. Arthropods vary greatly in size and shape and are among the most abundant of all animals. They have become successfully adapted to a wide variety of environments and live on land, in water, and in the air. The typical arthropod has a segmented body which is usually covered by a chitinous exoskeleton which, in some forms, contains additions of calcium carbonate. They are highly specialized and well-developed animals in which locomotion is by means of paired jointed appendages. Although the arthropods are of great importance in nature today, only a few groups are of importance to the paleontologist. Only two of these, the trilobites and the ostracodes, are discussed herein. Subphylum Trilobitomorpha The members of this subphylum are extinct arthropods which were most abundant during early Paleozoic time. CLASS TRILOBITA.— The trilobites are a group of exclusively marine arthropods which derive their name from the typical three-lobed appearance of their bodies (fig. 21a). The trilobite body is divided into a _central_ or _axial_ lobe and two _lateral_ lobes. The body of the animal was encased in a chitinous exoskeleton. The top part of this exterior covering, the _carapace_, is very thick, and it is this part of the trilobite that is usually preserved. [Illustration: Plate 34 FOSSIL ARTHROPODS] FOSSIL INSECT × 1 FOSSIL CRUSTACEANS ENOPLOCLYTIA × 1⅓ ASTACODES × ¾ NOTOPOCORYSTES × 2 OSTRACODES × 40 [Illustration: Fig. 21. Morphology and principal parts of trilobites.] Cephalon Thorax Pygidium Axial lobe Lateral lobes The body is also divided into three parts from front to back. Beginning at the front of the animal these divisions are the _cephalon_ or head, the _thorax_ or abdomen, and the _pygidium_ or tail (fig. 21a). The body segments of the thorax were arranged in such a manner as to permit the animal to roll up into a ball, and many trilobites are found in this position (fig. 21b). Trilobites first appeared in the Cambrian and were extinct by the end of the Permian. They occur sparingly in certain of the Paleozoic rocks of Texas and when found are likely to be fragmental and in a poor state of preservation. Subphylum Crustacea The crustaceans are the crabs, shrimp, crayfish, and ostracodes. Although not abundant, fossil crabs have been described from certain Cretaceous and Tertiary formations of the State (_Notopocorystes_, Pl. 34). However, the most useful and abundant crustacean fossils are the members of the class Ostracoda. CLASS OSTRACODA.—