See text Ptychopariida is a large, heterogeneous order of trilobite containing some of the most primitive species known.

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 Subclass Librostoma was recently erected to encompass several related orders, including Ptychopariida, Asaphida, Proetida, Harpetida, and possibly Phacopida.

Trilobites of the orders Proetida, Harpetida, and of the family Damesellidae were originally placed in Ptychopariida.