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.