Article: Hypostomes and ventral cephalic sutures in Cambrian trilobites

Restorations of the cephala of species of each of eighteen genera show the hypostome and cephalic sutures; new photographs are given of these features in Holmia, Bathynotus, Paradoxides, Fieldaspis, Ptychoparia, Conocoryphe, and Agraulos. It is considered that probably in all trilobites the tip of the upwardly directed anterior wing of the hypostome was situated close beneath the ridge formed on the internal surface of the cephalon by the axial furrow, in a position immediately in front of where the eye ridge or eye lobe met this furrow. This position of the hypostome may be observed in species in which the hypostome was attached either by a suture to the cephalic doublure, or fused to the rostral plate. In species in which the hypostome was detached from the cephalic doublure it is assumed that it was situated in a morphologically similar position. In forms in which it was attached, the hypostome was thus braced against the dorsal exoskeleton of the cephalon so that movement was not possible; such movement was probably restricted in detached forms. During development the close connection between anterior wing and a particular site in the axial furrow was maintained, hence the hypostome may have been detached in the early stages but attached in the holaspis, or vice versa. Fusion of hypostome to rostral plate in holaspids is known only in Cambrian trilobites. Progressive reduction in transverse width of the rostral plate, culminating in a median suture, is not known in an evolutionary series. Until more is known of the hypostome, cephalic doublure, and ventral sutures in Cambrian trilobites, these features will have only limited value in discriminating familial and higher taxa, compared with their importance in such characterization of post-Cambrian forms; this particularly applies to species having the hypostome detached.