Trilobites, of the class Trilobita, are a fossil group of extinct marine invertebrate arthropods. They have varying sizes ranging from a few millimetres to tens of centimetres. Trilobites flourished during the Cambrian and Ordovician and went extinct at the end of the Permian, with an evolutionary history of 300 million years. Trilobites had a flat body and a solid carapace with a soft peritoneal and appendages on the ventral side. The exoskeleton had two depressions on the back, which was divided longitudinally into three distinct lobes of head, chest and tail, from which the group gets its name. Trilobite fossils are mainly found in limestone, marl and shale. The famous trilobite fossils in Shangdong Province are known as Swallow Stone because the fossils appear like a flock of flying swallows. The ornamental types of trilobite fossils are called Bat Bug (Swallow Stone), Sichuan Bug, West Hunan Bug and Crown Bug.