Trilobites and ammonites are both index fossils, fossils that are used to identify different geologic periods. To be used as an index, a fossil has to be fairly common, have a worldwide distribution, and have evolved different body shapes fairly quickly (geologically speaking).
20" sterling silver chain
Two brass casts of genuine fossils - a trilobite on top with an ammonite below
Hand-fabricated, sterling silver setting with an antiqued, matte finish-the pendant is 1.75" long and the brass casts are backed in sterling silver