204 ARTHROPODA. TRILOBITA. 



of large facets. Thorax with eleven segments, pleurse 

 grooved. Pygidium variable. Ordovician to Devonian. 



Cheirurus. Head-shield semicircular, genal angles 

 pointed ; glabella convex, with three pairs of furrows, the 

 last uniting with the neck-furrow. Facial sutures con- 

 tinuous in front and ending on the external margin. 

 Eyes prominent. Thorax with usually eleven segments, 

 pleura grooved, and produced into spines. Pygidium 

 with four segments, lateral lobes with backwardly-directed 

 spines. Tremadoc to Devonian. 



Encrinurus. Head -shield covered with tubercles ; 

 glabella pyriform, eyes small, placed on short peduncles. 

 Facial sutures not continuous, ending on the lateral 

 margin. Thorax with eleven segments, pleuras ridged. 

 Pygidium narrow, triangular, pleurae bent backwards. 

 Bala to Upper Ludlow. 



Acidaspis. Head-shield broad, trilobation not well- 

 marked, genal angles spined ; glabella with a pair of 

 longitudinal furrows parallel to the axal furrows, and with 

 two or three lateral furrows. Eyes connected with the 

 glabella by a ridge. Facial sutures continuous in front 

 and ending on the lateral margins. Thorax with nine 

 or ten segments, pleurae produced into long spines. 

 Pygidium small, with long spines. Llandeilo Beds to 

 Devonian. 



Phillipsia. Body oval ; glabella with nearly parallel 

 sides, with two or three lateral furrows, the posterior one 

 being directed backwards and united to the deep neck- 

 furrow, thus cutting off a basal lobe. Eyes large, reniform. 

 Facial sutures not continuous in front ending on posterior 

 border. Thorax with seven segments, pleurse grooved. 



