三葉虫
Trilobites are a group of extinct marine arthropods. The first appearance of trilobites occurred in the Early Cambrian period (521 million years ago), and they disappeared in the end-Permian mass extinction about 252 million years ago.

Trilobites are characterized by a cephalon (head), thorax (body) and pygidium (posterior), and form a calcitic exoskeleton that is easily preserved as a [[fossil]].
trilobite