Middle Devonian Period- C. 400 Million Years Ago Order to which the fossil belongs: Phacopida Name of which the family belongs: Acastidae Informal name of the fossil: Trilobite Trilobites are a fossil group of extinct marine arachnomorph arthropods that form the class Trilobite, one of the earliest known groups of arthropods. The name originates from their division into three longitudinal lobes, consisting of a raised central lobe also known as the axis, with two slightly flatter pleural lobes either side. Trilobites also have a head shield known as the cephalon, and on the axial region of the head shield (glabella) they were cheeks either side and well-developed eyes. The thorax consists of up to thirty segments, each would have had a limb but these are rarely preserved. A trilobite would have probably roll up their external skeletons to protect themselves in defence. A nice example of a trilobite on a stone matrix with crisp definition. 40mm x 57mm length