CRUSTACEA. TRILOBITA 309 



Cybele. Similar to Encrinurus. Three pairs of more distinct 

 glabella furrows ; border continuous in front of the glabella ; genal 

 angles usually rounded; facial sutures continuous in front. Thorax 

 with 12 segments ; pleurae of the first 5 with blunt ends, those of 

 the remaining 7 produced into spines. Pygidium with 4 or 5 

 pleurae which bend sharply backwards and converge towards the 

 axis. Ordovician. Ex. C. verrucosa, Bala Beds. 



Lichas. Test covered with tubercles. Head-shield convex, 

 relatively small, with genal spines. Glabella broad, with a central 

 raised part, furrows directed backwards. Facial sutures pass from 

 the posterior to the anterior border. Cheeks and eyes small. 

 Thorax with 9 or 10 segments ; pleurae grooved, ending in rather 

 long spines. Pygidium large, showing 2 or 3 segments, lateral parts 

 produced into spines. Llandeilo to Wenlock. Ex. L. cmglicus, 

 "Wenlock. 



Fig. 133. Acidaspis prevosti, from the Silurian. Head-shield. (After 

 Barrande.) 1, 2, 3, first, second, and third glabella furrows (the 

 first usually indistinct) ; a, central part of the glabella ; c — b — n, 

 inner furrow of glabella ; c — v, neck-furrow ; d — v — x, axial furrow ; 

 Tc — .t, fixed cheek ; o, eye ; o — n, eye-line ; p, genal spines ; q, spines 

 from neck-ring ; r, neck-ring ; s — s', facial suture ; y, spines. En- 

 larged. 



Acidaspis (fig. 133). Head-shield broad, its trilobation not 

 well marked, with genal spines, and usually with spines at the 

 margin of the head; glabella with a pair of longitudinal furrows 

 parallel to the axial furrows, and with two or three lateral furrows. 

 Facial sutures start from the posterior margin just within the genal 

 angle and cut the front margin. Free cheeks large. Eyes con- 

 nected with the glabella by an eye-line. Thorax with 9 or 10 



