Complete exoskeletons of species of eight genera of Siluro-Devonian Scutelluinae show the morphology of the cephalon and thorax, particularly glabellar lobation, the wide inward extent of the doublure, and the development of fulcral processes and sockets, the latter more prominent on the external surface in stratigraphically younger species. The thorax of ten segments has axial rings with an articulating furrow and halfring as in most trilobites. The differences between the exoskeletons of Scutelluinae and Illaenidae are many, and imply anatomical differences. It is considered that Scutelluinae are related to the Ordovician Stygininae, and are to be united with them in Styginidae; a common origin for Styginidae and Illaenidae appears questionable.