=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size to the head; it consists of numerous segments which, however, may be indistinctly shown; the axis in some forms is obsolete.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_. [Illustration: 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.) ]