Fam. 7. Olenidae (Figs. [142], 143; 150, B, C).—The cephalic shield is larger than the pygidium. The glabella is either rectangular or parabolic. The facial suture passes from the posterior to the anterior margin. The palpebral lobes are of moderate or rather large size, and are connected by an eye-line with the front part of the glabella. The thorax includes from eleven (occasionally fewer) to eighteen segments with grooved pleurae. The pygidium is usually small, with from two to eight segments. Principally Cambrian. Genera: Ptychoparia, Angelina, Solenopleura, Sao, Agraulos (Arionellus), Ellipsocephalus, Protolenus, Olenus, Peltura, Acerocare, Eurycare, Ctenopyge, Leptoplastus, Triarthrus, Parabolina, Sphaerophthalmus, Parabolinella, Ceratopyge (position doubtful). Dikelocephalus is usually placed in the Olenidae, but perhaps belongs to a distinct family.
Fam. 8. Calymenidae (Figs. [136], 137).—The glabella is broadest behind. The facial suture starts at or near the genal angle—sometimes on the posterior border just inside the angle, sometimes on the lateral border just in front of the angle; the suture may be continuous with the other suture in front of the glabella, or may cut the anterior margin, beneath which it is connected with the other suture by means of a transverse suture (Fig. [137], B, D). The eyes are rather small. The thorax consists of thirteen segments with grooved pleurae; the pygidium of from six to fourteen segments. Ordovician to Devonian. Genera: Calymene, Synhomalonotus, Homalonotus.
Fig. [150].—A, Harpes ungula, Sternb., Ordovician. B, Ellipsocephalus hoffi, Scloth., Cambrian. C, Olenus truncatus, Brünn., Cambrian. (After Angelin.) D, Remopleurides radians, Barr., Ordovician. E, Conocoryphe sulzeri, Barr., Cambrian. F, Illaenus dalmanni, Volb., Ordovician. G, Proëtus bohemicus, Corda, Silurian, × 1½. H, Aeglina prisca, Barr., Ordovician, × 3. I, Phacops sternbergi, Barr., Devonian. (A, D, E, G, H, I, after Barrande; B, F, from Zittel; natural size except G, H.)
Fam. 9. Asaphidae (Fig. [150], F).—The body is oval and commonly rather large. The cephalic shield is large, with its glabella often indistinctly limited and the glabella-furrows often obscure. The facial suture starts from the posterior margin and usually cuts the anterior margin, but is sometimes continued in front of the glabella. The relative size of the fixed and free cheeks varies greatly. The eyes are of variable size. The thorax consists of eight or ten (sometimes fewer) segments; the pleurae are generally grooved, but sometimes plane. The pygidium is large, often being similar in form and size to the head; it consists of numerous segments which, however, may be indistinctly shown; the axis in some forms is obsolete. Upper Cambrian (Tremadoc) to Silurian; common in the Ordovician. Genera: Asaphus (sub-genera, Megalaspis, Asaphellus, Symphysurus, etc.), Ogygia, Barrandia, Niobe, Nileus, Illaenus, Bumastus, Stygina. Aeglina (Fig. [150], H) is usually placed in this family, but its systematic position is doubtful.
Fam. 10. Bronteidae.—The general form is similar to that of the Asaphidae. The glabella broadens rapidly in front, and is marked with furrows on each side, which are usually short, and may be indistinct. The facial suture passes from the posterior margin to the crescentic eye which is situated rather near the posterior border, and from thence to the anterior margin. There are ten thoracic segments with ridged pleurae. The pygidium is longer than the head, and has a very short axis, from which the furrows on the pleural part radiate. Ordovician to Devonian. Genus: Bronteus.
Fam. 11. Phacopidae (Figs. [138]; 150, I; 151, C).—The head and pygidium are of about the same size. The glabella is distinctly limited, and wider in front than behind, with a neck-furrow and three other furrows, of which some of the anterior may be indistinct or obsolete. The eyes are schizochroal and usually large. The facial suture begins at the lateral margin and unites with the suture of the other side in front of the glabella. There are eleven thoracic segments with grooved pleurae. The pygidium is usually large, with a distinct axis and many segments. Ordovician to Devonian. Genera: Phacops, Trimerocephalus, Acaste, Pterygometopus, Chasmops, Dalmanites, Cryphaeus.
Fig. [151].—A, Phillipsia gemmulifera, Phill., Carboniferous. B, Arethusina konincki, Barr., Ordovician. C, Dalmanites limulurus, Green, Silurian. (After Hall.) D, Cheirurus insignis, Beyr., Silurian. E, Deiphon forbesi, Barr., Silurian. F, Acidaspis dufrenoyi, Barr., Silurian. (A, B, from Zittel; D, E, F, after Barrande; natural size.)
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.