CRUSTACEA. TRILOBITA 307 



broad, pleurae neither grooved nor ridged. Pygidium large, semi- 

 circular, axis indistinct, segments not visible externally. Arenig to 

 Wenlock. Ex. /. davisi, I. bowmanni, Bala Beds. 



iEglina (fig. 124). Head-shield large ; glabella large, convex, 

 projecting beyond the margin in front. Cheeks narrow ; eyes very 

 large, occupying nearly all the free cheeks. Facial sutures discon- 

 tinuous, nearly parallel with the axis of the body. Thorax with 

 5 or 6 segments, axis broad, pleurae grooved. Pygidium rounded, 

 axis short. Arenig to Bala Beds. Ex. JE. binodosa, Arenig Beds. 



Bronteus. Head-shield large, semicircular, genal angles 

 pointed. Glabella expanding rapidly in front, with 3 lateral furrows 

 in some species, none in others. Facial sutures start from the 

 posterior border and are discontinuous in front. Free cheeks large ; 

 eyes crescentic, placed near the posterior border. Thorax with 10 

 segments, pleurae ridged. Pygidium very large, fan-shaped ; axis very 

 short; lateral lobes large, with radiating grooves. Bala Beds to 

 Devonian. Ex. B. Jtabellifer, Devonian. 



Phacops. Head-shield nearly semicircular ; glabella promi- 

 nent, broadest in front, with 3 or 4 furrows, which are sometimes 

 indistinct ; facial sutures commencing on the lateral borders of the 

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

 glabella. Eyes generally large, formed of large, distinct lenses. 

 Thorax with 11 segments, pleurae grooved. Pygidium variable. 

 Ordovician to Devonian. 



The species of Phacops are divided into a number of groups 

 which should be regarded as sub-genera, or perhaps genera ; some 

 of these are : — Phacops (restricted) : glabella inflated and expanded 

 in front, with the two anterior furrows obscure. Eyes large. No 

 genal spines. Ex. P. stokesi, Silurian. Trimerocephalus : glabella 

 furrows obscure or absent. Eyes small. No genal spines. Ex. 

 T. Icevis, Devonian. Acaste : glabella not much expanded in front, 

 all the furrows distinct. Ex. A. doicningice, Silurian. Chasmops : 

 glabella greatly expanded in front, two anterior furrows large, two 

 posterior very small. With genal spines. Ex. C. conophthalmus, 

 Bala Beds. Odontochile ( = Dalmanites) : glabella not much ex- 

 panded, all the furrows distinct. Genal spines long. Pleurae often 

 produced into spines. Ex. 0. caudatus, Silurian. 



20—2 



