 Even more pronounced is the situation that the frontal branches of the facial sutures end in each other, resulting in yoked free cheeks.This is known in "Triarthrus", and in the Phacopidae, but in that family the facial sutures are not functional, as can be concluded from the fact that free cheeks are not found separated from the cranidium.There are also two types of sutures in the dorsal surface connected to the compound eyes of trilobites.