CLASSIFICATION 249 



sists of thirteen segments with grooved pleurae ; the pygidium 

 of from six to fourteen segments. Ordovician to Devonian. 

 Genera: CaJyiiie)i(\ Synhomalonotus, Homalonotns. 



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 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 in some forms is obsolete.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, etc.), Ogygia, Barrandia, JViohe, 

 JVileus, Illaenus,Biimastus,Stygina. Acglina (Fig. 1 5 0,H) is usually 

 placed in this fixmily, 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 : Bronte us. 



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, Pterygomctopius, Chasniop)S, Dalmanites, Cryphaeus. 



