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 Cambrian trilobite Elrathia kingii from Utah, U.S.A. is a very common species found in most trilobite collections.