The fulcrate thorax of Stygina has the articulating furrow and a long articulating halfring, no pleural furrow, and the facetted outer portion of the pleura has the doublure terminating outside the fulcrum. In the type species the glabella is inflated and the axial furrows partially effaced; in other species inflation is not accompanied by such effacement. Distinctions between genera of Stygininae are based on cephalic characters, including the length (sag. and exs.) of the preglabellar area; the variability of these characters makes such distinctions difficult to define. Eobronteus has the forwardly-expanding glabella, three pairs of glabellar furrows, rostral plate and hypostome typical of Siluro-Devonian Scutelluinae. The relationship between the two subfamilies is shown by morphology and by ontogeny. It is doubted that Styginidae had an origin in common with Illaenidae; the characters suggesting a relationship between Nileus and illaenids are explored. To improve the classification of Trilobita, the search for Cambrian ancestors of Ordovician families needs to be intensified, and characters of the entire axial region should be taken into account. The impendent condition of the hypostome is homeomorphic and is of less significance in classification than the natant and conterminant conditions. Effacement of axial furrows of the cephalon is a trend displayed in Stygininae, and is another homeomorphic character that is not a reliable guide to relationship.