ARTHROPODA. TRILOBITA. 203 



Pygidium of about the same size as the head, rounded, 

 margin entire ; formed of numerous segments which are 

 usually visible on the axis only. Tremadoc to Bala Beds. 



Illaenus. Body oval, convex. Head-shield large, 

 semicircular ; glabella indistinctly marked, without furrows 

 externally, but within the test there are four pairs. Eyes 

 remote from one another. Facial sutures commence on 

 the posterior border, cut the anterior border in front of 

 eye and unite on the inferior surface. Thorax with 

 usually ten segments, pleurae not grooved or ridged. 

 Pygidium large, semicircular, axis rudimentary, segments 

 not visible externally. Arenig to Wenlock. 



JEglina. Head-shield large; glabella large, convex, 

 projecting beyond the margin in front. Cheeks narrow ; 

 eyes very large. Facial suture discontinuous, nearly 

 parallel to the axis of the body. Thorax with five or 

 six segments, pleurae grooved. Pygidium rounded, axis 

 reduced. Arenig to Bala Beds. 



Bronteus. Head-shield large, semicircular, genal 

 angles pointed. Glabella very wide in front, three lateral 

 furrows in some species, none in others. Facial sutures 

 discontinuous in front, eyes sickle- shaped, placed near 

 the posterior border. Thorax with ten segments, axis 

 narrow, pleurae ridged. Pygidium very large, fan-shaped, 

 axis short, lateral lobes large with radiating grooves. 

 Bala Beds to Devonian. 



Phacops. Head-shield nearly semicircular, glabella 

 prominent, broadest in front, with three or four furrows ; 

 facial sutures commencing on the lateral border of the 

 cheek in front of the genal angle, and continuous in front 

 of the glabella. Eyes generally large, granulated, formed 



