=Fam. 12. Cheiruridae= (Fig. 151, D, E).—The glabella is convex or inflated, and distinctly defined. The facial suture passes from the lateral to the front margin. The free cheeks are small, and the eyes usually rather small. There are from nine to eighteen (usually eleven) thoracic segments; the pleurae have ridges or grooves and free ends. The pygidium is small, consisting of from three to five segments often produced into spines. Upper Cambrian to Devonian. Genera: _Cheirurus_, _Deiphon_, _Placoparia_, _Sphaerexochus_, _Amphion_, _Staurocephalus_. =Fam. 13. Proëtidae= (Figs. 150, G; 151, A, B).—The body is rather small, and the head forms about a third of its entire length. The glabella is sharply defined, and its furrows are sometimes indistinct; the posterior furrow curves backward to the neck-furrow, thus limiting a basal lobe on each side of the glabella. The eyes are often large (Fig. 150, G); but in _Arethusina_ (Fig. 151, B), in which an eye-line is present, they are small. The facial sutures pass from the posterior to the anterior margin. The free cheeks are large. There are from eight to twenty-two thoracic segments with grooved pleurae. The pygidium is usually formed of numerous segments, and its margin is usually entire. Ordovician to Permian. Genera: _Proëtus_, _Arethusina_, _Cyphaspis_, _Phillipsia_, _Griffithides_, _Brachymetopus_, _Dechenella_.[203] =Fam. 14. Encrinuridae.=—The cephalic shield is ornamented with tubercles. The free cheeks are narrow, and the eyes very small. The facial suture extends from the lateral margin (or from the genal angle) to the anterior margin. There are from ten to twelve thoracic segments with ridged pleurae. On the axis of the pygidium numerous segments are seen, but usually fewer are indicated on the lateral parts. Ordovician and Silurian. Genera: _Encrinurus_, _Cybele_, _Dindymene_. =Fam. 15. Acidaspidae= (Fig. 151, F).—The cephalic shield is broad, with a spinose margin, genal spines, and sometimes spines on the neck-ring. The glabella has a longitudinal furrow on each side, due to the backward bending of the lateral furrows. The facial suture passes from the posterior border (near the genal angle) to the anterior border. The free cheeks are large; the eyes small. There are from eight to ten thoracic segments with ridged pleurae, which are produced into long backwardly directed spines. The pygidium is short, and is formed of two or three segments with long spines at the margin. Ordovician to Devonian. Genus: _Acidaspis_.