The ostracodes are small, bivalved, aquatic crustaceans which have the external appearance of small clams (Pl. 34). The remains of these tiny animals are so small that they are best studied under a low-power microscope, and because of their small size they are particularly useful to the micropaleontologist. Fossil ostracodes range from Ordovician to Recent in age and have been recorded in the Paleozoic, Mesozoic, and Cenozoic rocks of Texas. Their remains are particularly abundant in certain of the Cretaceous and Tertiary marine formations of the State. Phylum Echinodermata The echinoderms are a large group of exclusively marine animals, most of which exhibit a marked five-fold radial symmetry (Pls. 35, 36). Living echinoderms have well-developed nervous and digestive systems, a distinct body cavity, and are a relatively complex group of organisms. The typical echinoderm has a skeleton composed of numerous calcareous plates which are intricately fitted together and covered by a leathery outer skin (the _integument_). The echinoderm body often exhibits a typical star-shaped form, but some types may be heart-shaped, biscuit-shaped, or cucumber-shaped. Members of this phylum range from Cambrian to Recent in age and are abundant as fossils in many of the marine formations of Texas. The phylum Echinodermata has been divided into two subphyla, the Pelmatozoa (those forms that were attached to sea floor by a stem or a stalk) and the Eleutherozoa (the stemless unattached echinoderms). Subphylum Pelmatozoa These are echinoderms which are more or less permanently attached to the bottom of the sea by means of a stalk which is composed of slightly movable, calcareous, disk-like segments (fig. 23). Pelmatozoans range from Cambrian to Recent in age, and their fossilized remains are particularly abundant in Paleozoic rocks. The subphylum has been divided into several classes, but only three of these, the Cystoidea, Blastoidea, and Crinoidea, are discussed here. With the exception of the Crinoidea, all of the attached echinoderms are extinct. CLASS CYSTOIDEA.— These are primitive attached echinoderms which were relatively common in early Paleozoic time. The typical cystoid has a somewhat globular or sac-like _calyx_ (the main body skeleton) composed of numerous, irregularly arranged, calcareous plates (fig. 22b). The plates composing the calyx are usually perforated by pores or slits which were probably used in excretion or respiration. The calyx was attached to the sea bottom by a short stem. Cystoids range from Cambrian to Devonian in age and were especially abundant during Ordovician and Silurian time. Their remains are rare or absent in the rocks of Texas. [Illustration: Fig. 22. Two extinct attached echinoderms. (a) _Pentremites_ (Mississippian). (b) _Caryocrinites_ (Silurian).] CLASS BLASTOIDEA.—