Although they are extinct, these arthropods flourished in the sea, from the CAMBRIAN through the PERMIAN . They ranged in length from 1/2 in to 39 in.

The name, Trilobite, derives from their division into three longitudinal lobes: a slightly raised central lobe (the axis), with two flatter pleural lobes on either side.

They were divided into a head shield (cephalon), a thorax of up to 30 segments, and a tail shield (pygidium).

The axial region of the head shield (glabella) had cheeks on either sides and often well-developed eyes. Eachsegment of the thorax had limbs, but these are rarely preserved,

Trilobites could roll up (enroll) their external skeletons, probably for defense.