Phacopida ("Lens-face")'s an order of trilobite that lived from the Ordovician to the Devonian. It's made up of a morphologically diverse group of related suborders. The origin of the Phacopida order's uncertain. It comprises three suborders (Phacopina, Calymenina, and Cheirurina) which share a distinctive protaspis (developmental stage lacking segments) type. The Cheirurina and Calymenina retain a rostral plate (an apomorphy) but in virtually all Phacopina the free cheeks are yoked as a single… (More on Phacopida)