cephalon The anterior or head region of a trilobite (Trilobita), which consists of at least five fused segments and is generally semicircular in shape. The glabella (the raised median part of the cephalon) is very variable in size and structure and it is probable that in life the stomach lay beneath it. Between the lateral margins of the glabella and the facial suture (see CEPHALIC SUTURE) is the fixed cheek (fixigena) and between the facial suture and the lateral margin is the free cheek (librigena). The glabella and the fixed cheeks together are known as the ‘cranidium’. Above the visual surface of the eye is the palpebral lobe. In Cambrian trilobites the eyes are commonly connected to the glabella by narrow ocular ridges. The glabella may be indented by pairs of furrows and a transverse furrow separates the posterior occipital segment from the remainder of the glabella. In some families (e.g. the Trinucleidae) the border of the cephalon is developed into an extensive, pitted, cephalic fringe. The dorsal exoskeleton of the cephalon also continues on to the ventral surface as the ‘doublure’.

cephalon In some Arthropoda (e.g. Trilobitomorpha and Pycnogonida) the anterior portion of the body, or head, comprising fused segments.