Introduction: typically very spinose and densely sculptured trilobites.
Cephalon: convex; glabella tapering forward or subparallel, extending to anterior margin or nearly so, less complex lobation than in Lichoidea; eye ridges run from anterior end of glabella to palpebral lobe; opisthoparian sutures, often placed on sutural ridges; distinct notch in margin of free cheek adjacent to where anterior sutures cut cephalic margin; facial sutures secondarily lost in some genera; short genal spines typically present.
Thorax: 8 – 10 segments; tips of each bear 2 – 3 pairs of spines (anterior pair often difficult to see, ventrally directed); often with symmetrical row arrangements of pleural spines or tubercles. 
Pygidium: micropygous, short, transverse, with 2 – 3 axial rings (3rd often faint), one or more pairs of tubular border spines, the largest of which connected to first axial ring by prominent ridge.