Trilobita is a class of the phylum Arthropoda. All Arthropods have an external skeleton and many, hollow, jointed legs. Trilobites are divided lengthwise into three lobes, giving the group its name. The legs and antennae are seldom preserved. Trilobites are common from the Cambrian to the Carboniferous, becoming extinct in the Permian.

The specimen of Dalmanites myops from the Silurian of Ludlow, England, has moulted, so that only the cephalon (on the left) bearing the eyes and the pygidium or tail (on the right) is preserved.