VIII CLASSIFICATION 249 
sists of thirteen segments with grooved pleurae; the pygidium 
of from six to fourteen segments. Ordovician to Devonian. 
Genera: Calymene, Synhomalonotus, Homalonotus. 
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 formand size to the head; it consists of numer- 
ous 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in some forms is obsolete.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, Mega- 
laspis, Asaphellus, Symphysurus, ete.), 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. 
