[FOSSIL] INSECT × 1 FOSSIL CRUSTACEANS ENOPLOCLYTIA × 1⅓ ASTACODES × ¾ NOTOPOCORYSTES × 2 OSTRACODES × 40
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. 21]a). 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. 21]b).
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.—
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.