In trilobites (Trilobita), the structure that includes the facial and ventral cephalic sutures. The facial sutures situated on the dorsal part of the cephalon are of three main types: protoparian, where the suture runs from the anterior border of the cephalon to a position anterior to the genal angle; opisthoparian, where the suture runs from the anterior margin to cut the posterior margin posterior to the genal angle; and marginal, where the suture runs along the edge and is not visible on the upper surface. In one family the facial suture runs directly through the genal angle and is ‘gonatoparian’. On the ventral side of the cephalon the facial sutures are joined and continue as the ventral cephalic suture. An elongated rostral plate (see rostrum) in the anterior mid-line of the ventral surface has a rostral suture anterior to it and a hypostomal suture posterior to it. In some genera the lateral ventral sutures are combined as a single median suture.
Subjects: Earth Sciences and Geography.