Ptychopariida is a large, heterogeneous order of trilobite containing some of the most primitive species known. Many date to the Early Cambrian Period, but the order was extant through the Late Ordovician. Many of the offshoot species are difficult to classify in this single order.
Trilobites have facial sutures that run along the margin of the glabella and/or fixigena to the shoulder point where the cephalon meets the thorax. These sutures outline the cranidium, or the main, central part of the head that does not include the librigena (free cheeks). The eyes are medial along the glabella on the suture line (however, some species have no eyes). The fossils of the moults of trilobites can often be told from the fossils of the actual animals by whether or not the librigena are present. (The librigena are no longer attached in moults.) In Ptychopariida, short bladelike genal spines are often present on the tips of the librigena.
The thorax is large and is typically made up of eight or more segments. The thorax is usually much longer than the pygidium, which is usually small. In some species the pygidium is outlined with a flat border.
The Subclass Librostoma was recently created to encompass several related orders: Ptychopariida, Asaphida, Proetida, Harpetida, and possibly the Phacopida. These are now known as the "Librostome Orders". The Proetida and Harpetida were originally included in Order Ptychopariida.