Phacopida had 8 to 19 thoracic segments and are distinguishable by the expanded glabella, short or absent preglabellar area, and schizochroal (Phacopina) or holochroal (Cheirurina and Calymenina) eyes.

In Phacopida, these were retained, via delayed growth of these immature structures (post-displacement), into the adult form.

It comprises three suborders (Phacopina, Calymenina, and Cheirurina) which share a distinctive protaspis (unsegmented larva ) with three pairs of spines on its body.

The Cheirurina and Calymenina retain a rostral plate (an apomorphy) but in virtually all Phacopina the free cheeks are yoked as a single piece.

Phacopida was one of only two trilobite orders (along with the Proetida) to survive the Kellwasser event (Late Devonian extinction) at the Frasnian-Famennian boundary.